diff --git a/src/CoreShop/Bundle/CoreBundle/Validator/Constraints/CartStockAvailabilityValidator.php b/src/CoreShop/Bundle/CoreBundle/Validator/Constraints/CartStockAvailabilityValidator.php
index 0b53666c5e..519669a3bb 100644
--- a/src/CoreShop/Bundle/CoreBundle/Validator/Constraints/CartStockAvailabilityValidator.php
+++ b/src/CoreShop/Bundle/CoreBundle/Validator/Constraints/CartStockAvailabilityValidator.php
@@ -80,7 +80,7 @@ public function validate($value, Constraint $constraint): void
     private function getExistingCartItemQuantityFromCart(OrderInterface $cart, OrderItemInterface $cartItem): float
     {
         $product = $cartItem->getProduct();
-        $quantity = $cartItem->getDefaultUnitQuantity();
+        $quantity = $cartItem->getDefaultUnitQuantity() ?? 0;
 
         /**
          * @var OrderItemInterface $item