diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index f4fa0f42a0..cf22b6892b 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -6,7 +6,7 @@ includes:
     - vendor/phpstan/phpstan-symfony/rules.neon
 
 parameters:
-    level: 2
+    level: 3
 
     reportUnmatchedIgnoredErrors: false
 
diff --git a/src/Sylius/Bundle/AdminBundle/Controller/DashboardController.php b/src/Sylius/Bundle/AdminBundle/Controller/DashboardController.php
index 2ea795edf9..c1d283b5df 100644
--- a/src/Sylius/Bundle/AdminBundle/Controller/DashboardController.php
+++ b/src/Sylius/Bundle/AdminBundle/Controller/DashboardController.php
@@ -23,6 +23,7 @@ use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\Routing\RouterInterface;
 use Twig\Environment;
+use Webmozart\Assert\Assert;
 
 final class DashboardController
 {
@@ -71,9 +72,15 @@ final class DashboardController
     private function findChannelByCodeOrFindFirst(?string $channelCode): ?ChannelInterface
     {
         if (null !== $channelCode) {
-            return $this->channelRepository->findOneByCode($channelCode);
+            $channel = $this->channelRepository->findOneByCode($channelCode);
+            Assert::nullOrIsInstanceOf($channel, ChannelInterface::class);
+
+            return $channel;
         }
 
-        return $this->channelRepository->findOneBy([]);
+        $channel = $this->channelRepository->findOneBy([]);
+        Assert::nullOrIsInstanceOf($channel, ChannelInterface::class);
+
+        return $channel;
     }
 }
diff --git a/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailAwareCommandDataTransformer.php b/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailAwareCommandDataTransformer.php
index b81f3bacb6..ef498265e1 100644
--- a/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailAwareCommandDataTransformer.php
+++ b/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailAwareCommandDataTransformer.php
@@ -52,7 +52,10 @@ final class LoggedInCustomerEmailAwareCommandDataTransformer implements CommandD
         /** @var UserInterface|null $user */
         $user = $this->userContext->getUser();
         if ($user instanceof ShopUserInterface) {
-            return $user->getCustomer();
+            $customer = $user->getCustomer();
+            Assert::nullOrIsInstanceOf($customer, CustomerInterface::class);
+
+            return $customer;
         }
 
         return null;
diff --git a/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailIfNotSetAwareCommandDataTransformer.php b/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailIfNotSetAwareCommandDataTransformer.php
index 6a0ada1418..3797d921e4 100644
--- a/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailIfNotSetAwareCommandDataTransformer.php
+++ b/src/Sylius/Bundle/ApiBundle/DataTransformer/LoggedInCustomerEmailIfNotSetAwareCommandDataTransformer.php
@@ -57,7 +57,10 @@ final class LoggedInCustomerEmailIfNotSetAwareCommandDataTransformer implements
         /** @var UserInterface|null $user */
         $user = $this->userContext->getUser();
         if ($user instanceof ShopUserInterface) {
-            return $user->getCustomer();
+            $customer = $user->getCustomer();
+            Assert::nullOrIsInstanceOf($customer, CustomerInterface::class);
+
+            return $customer;
         }
 
         return null;
diff --git a/src/Sylius/Bundle/ApiBundle/Serializer/ShippingMethodNormalizer.php b/src/Sylius/Bundle/ApiBundle/Serializer/ShippingMethodNormalizer.php
index 7856c69302..6761d5ae6d 100644
--- a/src/Sylius/Bundle/ApiBundle/Serializer/ShippingMethodNormalizer.php
+++ b/src/Sylius/Bundle/ApiBundle/Serializer/ShippingMethodNormalizer.php
@@ -68,7 +68,7 @@ final class ShippingMethodNormalizer implements ContextAwareNormalizerInterface,
         }
 
         if (!isset($filters['tokenValue']) || !isset($filters['shipmentId'])) {
-            return;
+            return null;
         }
 
         /** @var ChannelInterface $channel */
diff --git a/src/Sylius/Bundle/CoreBundle/Form/Extension/CountryTypeExtension.php b/src/Sylius/Bundle/CoreBundle/Form/Extension/CountryTypeExtension.php
index 4721ba9c80..c4af70f963 100644
--- a/src/Sylius/Bundle/CoreBundle/Form/Extension/CountryTypeExtension.php
+++ b/src/Sylius/Bundle/CoreBundle/Form/Extension/CountryTypeExtension.php
@@ -80,9 +80,7 @@ final class CountryTypeExtension extends AbstractTypeExtension
         return Countries::getName($code);
     }
 
-    /**
-     * @return array|CountryInterface[]
-     */
+    /** @return string[] */
     private function getAvailableCountries(): array
     {
         $availableCountries = Countries::getNames();
diff --git a/src/Sylius/Bundle/CoreBundle/Form/Extension/LocaleTypeExtension.php b/src/Sylius/Bundle/CoreBundle/Form/Extension/LocaleTypeExtension.php
index e13a7550f6..939b810689 100644
--- a/src/Sylius/Bundle/CoreBundle/Form/Extension/LocaleTypeExtension.php
+++ b/src/Sylius/Bundle/CoreBundle/Form/Extension/LocaleTypeExtension.php
@@ -66,9 +66,7 @@ final class LocaleTypeExtension extends AbstractTypeExtension
         return Locales::getName($code);
     }
 
-    /**
-     * @return array|LocaleInterface[]
-     */
+    /** @return string[] */
     private function getAvailableLocales(): array
     {
         $availableLocales = Locales::getNames();
diff --git a/src/Sylius/Bundle/CoreBundle/Installer/Provider/DatabaseSetupCommandsProvider.php b/src/Sylius/Bundle/CoreBundle/Installer/Provider/DatabaseSetupCommandsProvider.php
index 8b02b1d862..9ed13841a6 100644
--- a/src/Sylius/Bundle/CoreBundle/Installer/Provider/DatabaseSetupCommandsProvider.php
+++ b/src/Sylius/Bundle/CoreBundle/Installer/Provider/DatabaseSetupCommandsProvider.php
@@ -21,6 +21,7 @@ use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\Console\Question\ConfirmationQuestion;
 use Symfony\Component\Console\Style\SymfonyStyle;
+use Webmozart\Assert\Assert;
 
 final class DatabaseSetupCommandsProvider implements DatabaseSetupCommandsProviderInterface
 {
@@ -131,6 +132,9 @@ final class DatabaseSetupCommandsProvider implements DatabaseSetupCommandsProvid
 
     private function getEntityManager(): EntityManagerInterface
     {
-        return $this->doctrineRegistry->getManager();
+        $objectManager = $this->doctrineRegistry->getManager();
+        Assert::isInstanceOf($objectManager, EntityManagerInterface::class);
+
+        return $objectManager;
     }
 }
diff --git a/src/Sylius/Bundle/CoreBundle/OAuth/UserProvider.php b/src/Sylius/Bundle/CoreBundle/OAuth/UserProvider.php
index 9fb2ffd5c9..9b5b68bf48 100644
--- a/src/Sylius/Bundle/CoreBundle/OAuth/UserProvider.php
+++ b/src/Sylius/Bundle/CoreBundle/OAuth/UserProvider.php
@@ -57,7 +57,10 @@ class UserProvider extends BaseUserProvider implements AccountConnectorInterface
         ]);
 
         if ($oauth instanceof UserOAuthInterface) {
-            return $oauth->getUser();
+            $user = $oauth->getUser();
+            Assert::isInstanceOf($user, UserInterface::class);
+
+            return $user;
         }
 
         if (null !== $response->getEmail()) {
@@ -79,6 +82,8 @@ class UserProvider extends BaseUserProvider implements AccountConnectorInterface
     }
 
     /**
+     * @return SyliusUserInterface&UserInterface
+     *
      * Ad-hoc creation of user.
      */
     private function createUserByOAuthUserResponse(UserResponseInterface $response): SyliusUserInterface
@@ -128,11 +133,14 @@ class UserProvider extends BaseUserProvider implements AccountConnectorInterface
     }
 
     /**
+     * @return SyliusUserInterface&UserInterface
+     *
      * Attach OAuth sign-in provider account to existing user.
      */
     private function updateUserByOAuthUserResponse(UserInterface $user, UserResponseInterface $response): SyliusUserInterface
     {
         /** @var SyliusUserInterface $user */
+        Assert::isInstanceOf($user, UserInterface::class);
         Assert::isInstanceOf($user, SyliusUserInterface::class);
 
         /** @var UserOAuthInterface $oauth */
diff --git a/src/Sylius/Bundle/CoreBundle/Provider/FlashBagProvider.php b/src/Sylius/Bundle/CoreBundle/Provider/FlashBagProvider.php
index cde2e65346..e451872681 100644
--- a/src/Sylius/Bundle/CoreBundle/Provider/FlashBagProvider.php
+++ b/src/Sylius/Bundle/CoreBundle/Provider/FlashBagProvider.php
@@ -16,6 +16,7 @@ namespace Sylius\Bundle\CoreBundle\Provider;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface;
 use Symfony\Component\HttpFoundation\Session\SessionInterface;
+use Webmozart\Assert\Assert;
 
 final class FlashBagProvider
 {
@@ -27,9 +28,15 @@ final class FlashBagProvider
         }
 
         if ($requestStackSessionOrFlashBag instanceof SessionInterface) {
-            return $requestStackSessionOrFlashBag->getBag('flashes');
+            $flashBag = $requestStackSessionOrFlashBag->getBag('flashes');
+            Assert::isInstanceOf($flashBag, FlashBagInterface::class);
+
+            return $flashBag;
         }
 
-        return $requestStackSessionOrFlashBag->getSession()->getBag('flashes');
+        $flashBag = $requestStackSessionOrFlashBag->getSession()->getBag('flashes');
+        Assert::isInstanceOf($flashBag, FlashBagInterface::class);
+
+        return $flashBag;
     }
 }
diff --git a/src/Sylius/Bundle/PayumBundle/Request/GetStatus.php b/src/Sylius/Bundle/PayumBundle/Request/GetStatus.php
index 7e2343388a..219979199c 100644
--- a/src/Sylius/Bundle/PayumBundle/Request/GetStatus.php
+++ b/src/Sylius/Bundle/PayumBundle/Request/GetStatus.php
@@ -20,6 +20,7 @@ class GetStatus extends BaseGetStatus
 {
     /**
      * @psalm-suppress NonInvariantDocblockPropertyType
+     * @phpstan-ignore-next-line
      *
      * @var string
      */
diff --git a/src/Sylius/Bundle/ProductBundle/Form/EventSubscriber/GenerateProductVariantsSubscriber.php b/src/Sylius/Bundle/ProductBundle/Form/EventSubscriber/GenerateProductVariantsSubscriber.php
index d625216ca8..9cd3faa3d9 100644
--- a/src/Sylius/Bundle/ProductBundle/Form/EventSubscriber/GenerateProductVariantsSubscriber.php
+++ b/src/Sylius/Bundle/ProductBundle/Form/EventSubscriber/GenerateProductVariantsSubscriber.php
@@ -59,9 +59,15 @@ final class GenerateProductVariantsSubscriber implements EventSubscriberInterfac
     private function getFlashBag(): FlashBagInterface
     {
         if ($this->requestStackOrSession instanceof RequestStack) {
-            return $this->requestStackOrSession->getSession()->getBag('flashes');
+            $flashBag = $this->requestStackOrSession->getSession()->getBag('flashes');
+            Assert::isInstanceOf($flashBag, FlashBagInterface::class);
+
+            return $flashBag;
         }
 
-        return $this->requestStackOrSession->getBag('flashes');
+        $flashBag = $this->requestStackOrSession->getBag('flashes');
+        Assert::isInstanceOf($flashBag, FlashBagInterface::class);
+
+        return $flashBag;
     }
 }
diff --git a/src/Sylius/Bundle/ShopBundle/Router/LocaleStrippingRouter.php b/src/Sylius/Bundle/ShopBundle/Router/LocaleStrippingRouter.php
index 61b0071339..1289bb85d0 100644
--- a/src/Sylius/Bundle/ShopBundle/Router/LocaleStrippingRouter.php
+++ b/src/Sylius/Bundle/ShopBundle/Router/LocaleStrippingRouter.php
@@ -57,11 +57,14 @@ final class LocaleStrippingRouter implements RouterInterface, WarmableInterface
         return $this->router->getRouteCollection();
     }
 
-    public function warmUp($cacheDir): void
+    /** @return string[] */
+    public function warmUp($cacheDir): array
     {
         if ($this->router instanceof WarmableInterface) {
-            $this->router->warmUp($cacheDir);
+            return $this->router->warmUp($cacheDir);
         }
+
+        return [];
     }
 
     private function removeUnusedQueryArgument(string $url, string $key, string $value): string
diff --git a/src/Sylius/Bundle/UserBundle/Command/AbstractRoleCommand.php b/src/Sylius/Bundle/UserBundle/Command/AbstractRoleCommand.php
index a550f1de9f..adf94c9320 100644
--- a/src/Sylius/Bundle/UserBundle/Command/AbstractRoleCommand.php
+++ b/src/Sylius/Bundle/UserBundle/Command/AbstractRoleCommand.php
@@ -123,7 +123,10 @@ abstract class AbstractRoleCommand extends ContainerAwareCommand
     {
         $class = $this->getUserModelClass($userType);
 
-        return $this->getEntityManager($userType)->getRepository($class);
+        $userRepository = $this->getEntityManager($userType)->getRepository($class);
+        Assert::isInstanceOf($userRepository, UserRepositoryInterface::class);
+
+        return $userRepository;
     }
 
     protected function getAvailableUserTypes(): array
diff --git a/src/Sylius/Bundle/UserBundle/Provider/EmailProvider.php b/src/Sylius/Bundle/UserBundle/Provider/EmailProvider.php
index 5d0db0180b..4ec378d65d 100644
--- a/src/Sylius/Bundle/UserBundle/Provider/EmailProvider.php
+++ b/src/Sylius/Bundle/UserBundle/Provider/EmailProvider.php
@@ -14,11 +14,15 @@ declare(strict_types=1);
 namespace Sylius\Bundle\UserBundle\Provider;
 
 use Symfony\Component\Security\Core\User\UserInterface;
+use Webmozart\Assert\Assert;
 
 class EmailProvider extends AbstractUserProvider
 {
     protected function findUser(string $uniqueIdentifier): ?UserInterface
     {
-        return $this->userRepository->findOneByEmail($uniqueIdentifier);
+        $user = $this->userRepository->findOneByEmail($uniqueIdentifier);
+        Assert::nullOrIsInstanceOf($user, UserInterface::class);
+
+        return $user;
     }
 }
diff --git a/src/Sylius/Bundle/UserBundle/Provider/UsernameOrEmailProvider.php b/src/Sylius/Bundle/UserBundle/Provider/UsernameOrEmailProvider.php
index ebdc34c4be..11d405db2e 100644
--- a/src/Sylius/Bundle/UserBundle/Provider/UsernameOrEmailProvider.php
+++ b/src/Sylius/Bundle/UserBundle/Provider/UsernameOrEmailProvider.php
@@ -14,15 +14,22 @@ declare(strict_types=1);
 namespace Sylius\Bundle\UserBundle\Provider;
 
 use Symfony\Component\Security\Core\User\UserInterface;
+use Webmozart\Assert\Assert;
 
 class UsernameOrEmailProvider extends AbstractUserProvider
 {
     protected function findUser(string $uniqueIdentifier): ?UserInterface
     {
         if (filter_var($uniqueIdentifier, \FILTER_VALIDATE_EMAIL)) {
-            return $this->userRepository->findOneByEmail($uniqueIdentifier);
+            $user = $this->userRepository->findOneByEmail($uniqueIdentifier);
+            Assert::nullOrIsInstanceOf($user, UserInterface::class);
+
+            return $user;
         }
 
-        return $this->userRepository->findOneBy(['usernameCanonical' => $uniqueIdentifier]);
+        $user = $this->userRepository->findOneBy(['usernameCanonical' => $uniqueIdentifier]);
+        Assert::nullOrIsInstanceOf($user, UserInterface::class);
+
+        return $user;
     }
 }
diff --git a/src/Sylius/Bundle/UserBundle/Provider/UsernameProvider.php b/src/Sylius/Bundle/UserBundle/Provider/UsernameProvider.php
index 5c941ae0c2..40be3b1249 100644
--- a/src/Sylius/Bundle/UserBundle/Provider/UsernameProvider.php
+++ b/src/Sylius/Bundle/UserBundle/Provider/UsernameProvider.php
@@ -14,11 +14,15 @@ declare(strict_types=1);
 namespace Sylius\Bundle\UserBundle\Provider;
 
 use Symfony\Component\Security\Core\User\UserInterface;
+use Webmozart\Assert\Assert;
 
 class UsernameProvider extends AbstractUserProvider
 {
     protected function findUser(string $uniqueIdentifier): ?UserInterface
     {
-        return $this->userRepository->findOneBy(['usernameCanonical' => $uniqueIdentifier]);
+        $user = $this->userRepository->findOneBy(['usernameCanonical' => $uniqueIdentifier]);
+        Assert::nullOrIsInstanceOf($user, UserInterface::class);
+
+        return $user;
     }
 }
diff --git a/src/Sylius/Component/Channel/Context/SingleChannelContext.php b/src/Sylius/Component/Channel/Context/SingleChannelContext.php
index 1b02fc9b1f..3fe3210678 100644
--- a/src/Sylius/Component/Channel/Context/SingleChannelContext.php
+++ b/src/Sylius/Component/Channel/Context/SingleChannelContext.php
@@ -15,6 +15,7 @@ namespace Sylius\Component\Channel\Context;
 
 use Sylius\Component\Channel\Model\ChannelInterface;
 use Sylius\Component\Channel\Repository\ChannelRepositoryInterface;
+use Webmozart\Assert\Assert;
 
 final class SingleChannelContext implements ChannelContextInterface
 {
@@ -29,7 +30,9 @@ final class SingleChannelContext implements ChannelContextInterface
         if (1 !== count($channels)) {
             throw new ChannelNotFoundException();
         }
+        $channel = reset($channels);
+        Assert::isInstanceOf($channel, ChannelInterface::class);
 
-        return current($channels);
+        return $channel;
     }
 }
diff --git a/src/Sylius/Component/Core/Factory/ChannelFactory.php b/src/Sylius/Component/Core/Factory/ChannelFactory.php
index 44d04b2e82..17e8fa4fad 100644
--- a/src/Sylius/Component/Core/Factory/ChannelFactory.php
+++ b/src/Sylius/Component/Core/Factory/ChannelFactory.php
@@ -17,6 +17,7 @@ use Sylius\Component\Channel\Factory\ChannelFactoryInterface;
 use Sylius\Component\Channel\Model\ChannelInterface;
 use Sylius\Component\Core\Model\ChannelInterface as CoreChannelInterface;
 use Sylius\Component\Resource\Factory\FactoryInterface;
+use Webmozart\Assert\Assert;
 
 final class ChannelFactory implements ChannelFactoryInterface
 {
@@ -36,13 +37,11 @@ final class ChannelFactory implements ChannelFactoryInterface
         return $channel;
     }
 
-    /**
-     * @return CoreChannelInterface
-     */
     public function createNamed(string $name): ChannelInterface
     {
         $channel = $this->createNew();
         $channel->setName($name);
+        Assert::isInstanceOf($channel, ChannelInterface::class);
 
         return $channel;
     }
diff --git a/src/Sylius/Component/Core/Model/Address.php b/src/Sylius/Component/Core/Model/Address.php
index 595b00adb2..eeab5605b1 100644
--- a/src/Sylius/Component/Core/Model/Address.php
+++ b/src/Sylius/Component/Core/Model/Address.php
@@ -15,6 +15,7 @@ namespace Sylius\Component\Core\Model;
 
 use Sylius\Component\Addressing\Model\Address as BaseAddress;
 use Sylius\Component\Customer\Model\CustomerInterface as BaseCustomerInterface;
+use Webmozart\Assert\Assert;
 
 class Address extends BaseAddress implements AddressInterface
 {
@@ -28,6 +29,7 @@ class Address extends BaseAddress implements AddressInterface
 
     public function setCustomer(?BaseCustomerInterface $customer): void
     {
+        Assert::nullOrIsInstanceOf($customer, CustomerInterface::class);
         $this->customer = $customer;
     }
 }
diff --git a/src/Sylius/Component/Core/Model/CatalogPromotion.php b/src/Sylius/Component/Core/Model/CatalogPromotion.php
index 3d0ecefc38..5c0ce741b0 100644
--- a/src/Sylius/Component/Core/Model/CatalogPromotion.php
+++ b/src/Sylius/Component/Core/Model/CatalogPromotion.php
@@ -18,6 +18,7 @@ use Doctrine\Common\Collections\Collection;
 use Sylius\Component\Channel\Model\ChannelInterface as BaseChannelInterface;
 use Sylius\Component\Promotion\Model\CatalogPromotion as BaseCatalogPromotion;
 use Sylius\Component\Promotion\Model\CatalogPromotionTranslation;
+use Webmozart\Assert\Assert;
 
 class CatalogPromotion extends BaseCatalogPromotion implements CatalogPromotionInterface
 {
@@ -41,6 +42,7 @@ class CatalogPromotion extends BaseCatalogPromotion implements CatalogPromotionI
      */
     public function getChannels(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->channels;
     }
 
diff --git a/src/Sylius/Component/Core/Model/Order.php b/src/Sylius/Component/Core/Model/Order.php
index 7b5a64f820..c5a40b4997 100644
--- a/src/Sylius/Component/Core/Model/Order.php
+++ b/src/Sylius/Component/Core/Model/Order.php
@@ -128,6 +128,7 @@ class Order extends BaseOrder implements OrderInterface
 
     public function setChannel(?BaseChannelInterface $channel): void
     {
+        Assert::isInstanceOf($channel, ChannelInterface::class);
         $this->channel = $channel;
     }
 
@@ -208,6 +209,7 @@ class Order extends BaseOrder implements OrderInterface
      */
     public function getPayments(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->payments;
     }
 
diff --git a/src/Sylius/Component/Core/Model/OrderInterface.php b/src/Sylius/Component/Core/Model/OrderInterface.php
index b54aa9a034..a468acf32e 100644
--- a/src/Sylius/Component/Core/Model/OrderInterface.php
+++ b/src/Sylius/Component/Core/Model/OrderInterface.php
@@ -132,6 +132,8 @@ interface OrderInterface extends
      * @psalm-return Collection<array-key, OrderItemInterface>
      *
      * @psalm-suppress ImplementedReturnTypeMismatch
+     *
+     * @phpstan-ignore-next-line
      */
     public function getItems(): Collection;
 
diff --git a/src/Sylius/Component/Core/Model/OrderItem.php b/src/Sylius/Component/Core/Model/OrderItem.php
index 9817c5a96d..63cd023813 100644
--- a/src/Sylius/Component/Core/Model/OrderItem.php
+++ b/src/Sylius/Component/Core/Model/OrderItem.php
@@ -54,7 +54,10 @@ class OrderItem extends BaseOrderItem implements OrderItemInterface
 
     public function getProduct(): ?ProductInterface
     {
-        return $this->variant->getProduct();
+        $product = $this->variant->getProduct();
+        Assert::nullOrIsInstanceOf($product, ProductInterface::class);
+
+        return $product;
     }
 
     public function getProductName(): ?string
diff --git a/src/Sylius/Component/Core/Model/OrderItemUnit.php b/src/Sylius/Component/Core/Model/OrderItemUnit.php
index 8a650ab20d..9bcc60130e 100644
--- a/src/Sylius/Component/Core/Model/OrderItemUnit.php
+++ b/src/Sylius/Component/Core/Model/OrderItemUnit.php
@@ -41,6 +41,7 @@ class OrderItemUnit extends BaseOrderItemUnit implements OrderItemUnitInterface
 
     public function setShipment(?BaseShipmentInterface $shipment): void
     {
+        Assert::nullOrIsInstanceOf($shipment, ShipmentInterface::class);
         $this->shipment = $shipment;
     }
 
diff --git a/src/Sylius/Component/Core/Model/Payment.php b/src/Sylius/Component/Core/Model/Payment.php
index 36ddd54453..96189ccdc8 100644
--- a/src/Sylius/Component/Core/Model/Payment.php
+++ b/src/Sylius/Component/Core/Model/Payment.php
@@ -15,6 +15,7 @@ namespace Sylius\Component\Core\Model;
 
 use Sylius\Component\Order\Model\OrderInterface as BaseOrderInterface;
 use Sylius\Component\Payment\Model\Payment as BasePayment;
+use Webmozart\Assert\Assert;
 
 class Payment extends BasePayment implements PaymentInterface
 {
@@ -23,6 +24,8 @@ class Payment extends BasePayment implements PaymentInterface
 
     public function getOrder(): ?BaseOrderInterface
     {
+        Assert::isInstanceOf($this->order, OrderInterface::class);
+
         return $this->order;
     }
 
diff --git a/src/Sylius/Component/Core/Model/Product.php b/src/Sylius/Component/Core/Model/Product.php
index ac18fb03ec..aa21b19347 100644
--- a/src/Sylius/Component/Core/Model/Product.php
+++ b/src/Sylius/Component/Core/Model/Product.php
@@ -149,6 +149,7 @@ class Product extends BaseProduct implements ProductInterface, ReviewableProduct
      */
     public function getChannels(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->channels;
     }
 
@@ -272,7 +273,10 @@ class Product extends BaseProduct implements ProductInterface, ReviewableProduct
      */
     public function getTranslation(?string $locale = null): TranslationInterface
     {
-        return parent::getTranslation($locale);
+        $productTranslation = parent::getTranslation($locale);
+        Assert::isInstanceOf($productTranslation, ProductTranslationInterface::class);
+
+        return $productTranslation;
     }
 
     /**
diff --git a/src/Sylius/Component/Core/Model/ProductVariant.php b/src/Sylius/Component/Core/Model/ProductVariant.php
index 6921b3f0c7..3a6958ef4d 100644
--- a/src/Sylius/Component/Core/Model/ProductVariant.php
+++ b/src/Sylius/Component/Core/Model/ProductVariant.php
@@ -19,6 +19,7 @@ use Doctrine\Common\Comparable;
 use Sylius\Component\Product\Model\ProductVariant as BaseVariant;
 use Sylius\Component\Shipping\Model\ShippingCategoryInterface;
 use Sylius\Component\Taxation\Model\TaxCategoryInterface;
+use Webmozart\Assert\Assert;
 
 class ProductVariant extends BaseVariant implements ProductVariantInterface, Comparable, \Stringable
 {
@@ -291,6 +292,7 @@ class ProductVariant extends BaseVariant implements ProductVariantInterface, Com
      */
     public function getImages(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->images;
     }
 
@@ -300,9 +302,13 @@ class ProductVariant extends BaseVariant implements ProductVariantInterface, Com
      */
     public function getImagesByType(string $type): Collection
     {
-        return $this->images->filter(function (ProductImageInterface $image) use ($type): bool {
+        /** @var Collection<array-key, ImageInterface> $imagesByType */
+        $imagesByType = $this->images->filter(function (ProductImageInterface $image) use ($type): bool {
             return $type === $image->getType();
         });
+        Assert::allIsInstanceOf($imagesByType, ImageInterface::class);
+
+        return $imagesByType;
     }
 
     public function hasImages(): bool
diff --git a/src/Sylius/Component/Core/Model/Promotion.php b/src/Sylius/Component/Core/Model/Promotion.php
index 689a3c9cf8..fa5071be7b 100644
--- a/src/Sylius/Component/Core/Model/Promotion.php
+++ b/src/Sylius/Component/Core/Model/Promotion.php
@@ -41,6 +41,7 @@ class Promotion extends BasePromotion implements PromotionInterface
      */
     public function getChannels(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->channels;
     }
 
diff --git a/src/Sylius/Component/Core/Model/Shipment.php b/src/Sylius/Component/Core/Model/Shipment.php
index 7d9af9acc6..cdef126b58 100644
--- a/src/Sylius/Component/Core/Model/Shipment.php
+++ b/src/Sylius/Component/Core/Model/Shipment.php
@@ -18,6 +18,7 @@ use Doctrine\Common\Collections\Collection;
 use Sylius\Component\Order\Model\AdjustmentInterface as BaseAdjustmentInterface;
 use Sylius\Component\Order\Model\OrderInterface as BaseOrderInterface;
 use Sylius\Component\Shipping\Model\Shipment as BaseShipment;
+use Webmozart\Assert\Assert;
 
 class Shipment extends BaseShipment implements ShipmentInterface
 {
@@ -44,6 +45,7 @@ class Shipment extends BaseShipment implements ShipmentInterface
 
     public function getOrder(): ?BaseOrderInterface
     {
+        Assert::nullOrIsInstanceOf($this->order, OrderInterface::class);
         return $this->order;
     }
 
diff --git a/src/Sylius/Component/Core/Model/ShippingMethod.php b/src/Sylius/Component/Core/Model/ShippingMethod.php
index 4d6e10ca9c..2d7770d85a 100644
--- a/src/Sylius/Component/Core/Model/ShippingMethod.php
+++ b/src/Sylius/Component/Core/Model/ShippingMethod.php
@@ -70,6 +70,7 @@ class ShippingMethod extends BaseShippingMethod implements ShippingMethodInterfa
      */
     public function getChannels(): Collection
     {
+        /** @phpstan-ignore-next-line */
         return $this->channels;
     }
 
diff --git a/src/Sylius/Component/Core/Payment/Provider/OrderPaymentProvider.php b/src/Sylius/Component/Core/Payment/Provider/OrderPaymentProvider.php
index 5d15c41b24..abaf2d7a69 100644
--- a/src/Sylius/Component/Core/Payment/Provider/OrderPaymentProvider.php
+++ b/src/Sylius/Component/Core/Payment/Provider/OrderPaymentProvider.php
@@ -23,6 +23,7 @@ use Sylius\Component\Payment\Factory\PaymentFactoryInterface;
 use Sylius\Component\Payment\PaymentTransitions;
 use Sylius\Component\Payment\Resolver\DefaultPaymentMethodResolverInterface;
 use Sylius\Component\Resource\StateMachine\StateMachineInterface;
+use Webmozart\Assert\Assert;
 
 final class OrderPaymentProvider implements OrderPaymentProviderInterface
 {
@@ -72,8 +73,10 @@ final class OrderPaymentProvider implements OrderPaymentProviderInterface
     {
         try {
             $payment->setOrder($order);
+            $paymentMethod = $this->defaultPaymentMethodResolver->getDefaultPaymentMethod($payment);
+            Assert::isInstanceOf($paymentMethod, PaymentMethodInterface::class);
 
-            return $this->defaultPaymentMethodResolver->getDefaultPaymentMethod($payment);
+            return $paymentMethod;
         } catch (UnresolvedDefaultPaymentMethodException) {
             return null;
         }
diff --git a/src/Sylius/Component/Core/StateResolver/OrderPaymentStateResolver.php b/src/Sylius/Component/Core/StateResolver/OrderPaymentStateResolver.php
index 480f187bbc..17cdf9c86b 100644
--- a/src/Sylius/Component/Core/StateResolver/OrderPaymentStateResolver.php
+++ b/src/Sylius/Component/Core/StateResolver/OrderPaymentStateResolver.php
@@ -122,8 +122,12 @@ final class OrderPaymentStateResolver implements StateResolverInterface
      */
     private function getPaymentsWithState(OrderInterface $order, string $state): Collection
     {
-        return $order->getPayments()->filter(function (PaymentInterface $payment) use ($state) {
+        /** @var Collection<array-key, PaymentInterface> $payments */
+        $payments = $order->getPayments()->filter(function (PaymentInterface $payment) use ($state) {
             return $state === $payment->getState();
         });
+        Assert::allIsInstanceOf($payments, PaymentInterface::class);
+
+        return $payments;
     }
 }
diff --git a/src/Sylius/Component/Core/spec/Model/OrderItemUnitSpec.php b/src/Sylius/Component/Core/spec/Model/OrderItemUnitSpec.php
index 3f6fd7b788..06409ab3f1 100644
--- a/src/Sylius/Component/Core/spec/Model/OrderItemUnitSpec.php
+++ b/src/Sylius/Component/Core/spec/Model/OrderItemUnitSpec.php
@@ -19,9 +19,9 @@ use Sylius\Component\Core\Model\AdjustmentInterface;
 use Sylius\Component\Core\Model\OrderItemInterface;
 use Sylius\Component\Core\Model\OrderItemUnitInterface;
 use Sylius\Component\Core\Model\ProductVariantInterface;
+use Sylius\Component\Core\Model\ShipmentInterface;
 use Sylius\Component\Inventory\Model\InventoryUnitInterface;
 use Sylius\Component\Order\Model\OrderItemUnit as BaseOrderItemUnit;
-use Sylius\Component\Shipping\Model\ShipmentInterface;
 use Sylius\Component\Shipping\Model\ShipmentUnitInterface;
 
 final class OrderItemUnitSpec extends ObjectBehavior
diff --git a/src/Sylius/Component/Payment/Resolver/DefaultPaymentMethodResolver.php b/src/Sylius/Component/Payment/Resolver/DefaultPaymentMethodResolver.php
index 591161cfe8..f20ce873c1 100644
--- a/src/Sylius/Component/Payment/Resolver/DefaultPaymentMethodResolver.php
+++ b/src/Sylius/Component/Payment/Resolver/DefaultPaymentMethodResolver.php
@@ -17,6 +17,7 @@ use Sylius\Component\Payment\Exception\UnresolvedDefaultPaymentMethodException;
 use Sylius\Component\Payment\Model\PaymentInterface;
 use Sylius\Component\Payment\Model\PaymentMethodInterface;
 use Sylius\Component\Payment\Repository\PaymentMethodRepositoryInterface;
+use Webmozart\Assert\Assert;
 
 final class DefaultPaymentMethodResolver implements DefaultPaymentMethodResolverInterface
 {
@@ -33,7 +34,9 @@ final class DefaultPaymentMethodResolver implements DefaultPaymentMethodResolver
         if (empty($paymentMethods)) {
             throw new UnresolvedDefaultPaymentMethodException();
         }
+        $paymentMethod = $paymentMethods[0];
+        Assert::isInstanceOf($paymentMethod, PaymentMethodInterface::class);
 
-        return $paymentMethods[0];
+        return $paymentMethod;
     }
 }
diff --git a/src/Sylius/Component/Shipping/Resolver/DefaultShippingMethodResolver.php b/src/Sylius/Component/Shipping/Resolver/DefaultShippingMethodResolver.php
index 4b72dc48fb..ff71877f87 100644
--- a/src/Sylius/Component/Shipping/Resolver/DefaultShippingMethodResolver.php
+++ b/src/Sylius/Component/Shipping/Resolver/DefaultShippingMethodResolver.php
@@ -17,6 +17,7 @@ use Sylius\Component\Shipping\Exception\UnresolvedDefaultShippingMethodException
 use Sylius\Component\Shipping\Model\ShipmentInterface;
 use Sylius\Component\Shipping\Model\ShippingMethodInterface;
 use Sylius\Component\Shipping\Repository\ShippingMethodRepositoryInterface;
+use Webmozart\Assert\Assert;
 
 final class DefaultShippingMethodResolver implements DefaultShippingMethodResolverInterface
 {
@@ -30,7 +31,9 @@ final class DefaultShippingMethodResolver implements DefaultShippingMethodResolv
         if (empty($shippingMethods)) {
             throw new UnresolvedDefaultShippingMethodException();
         }
+        $shippingMethod = $shippingMethods[0];
+        Assert::isInstanceOf($shippingMethod, ShippingMethodInterface::class);
 
-        return $shippingMethods[0];
+        return $shippingMethod;
     }
 }