diff --git a/app/code/Magento/CatalogInventory/Observer/ProcessInventoryDataObserver.php b/app/code/Magento/CatalogInventory/Observer/ProcessInventoryDataObserver.php
index e473f714bd21d..cea19c098b928 100644
--- a/app/code/Magento/CatalogInventory/Observer/ProcessInventoryDataObserver.php
+++ b/app/code/Magento/CatalogInventory/Observer/ProcessInventoryDataObserver.php
@@ -97,7 +97,7 @@ private function prepareQuantityAndStockStatus(StockItemInterface $stockItem, ar
             ) {
                 unset($quantityAndStockStatus['is_in_stock']);
             }
-            if (isset($quantityAndStockStatus['qty'])
+            if (array_key_exists('qty', $quantityAndStockStatus)
                 && $stockItem->getQty() == $quantityAndStockStatus['qty']
             ) {
                 unset($quantityAndStockStatus['qty']);
diff --git a/dev/tests/api-functional/testsuite/Magento/CatalogInventory/Api/ProductRepositoryInterfaceTest.php b/dev/tests/api-functional/testsuite/Magento/CatalogInventory/Api/ProductRepositoryInterfaceTest.php
index 9478eca88a3fa..5422362afd73c 100644
--- a/dev/tests/api-functional/testsuite/Magento/CatalogInventory/Api/ProductRepositoryInterfaceTest.php
+++ b/dev/tests/api-functional/testsuite/Magento/CatalogInventory/Api/ProductRepositoryInterfaceTest.php
@@ -23,8 +23,9 @@ class ProductRepositoryInterfaceTest extends WebapiAbstract
     const KEY_PRODUCT_ID = StockStatusInterface::PRODUCT_ID;
     const KEY_CUSTOM_ATTRIBUTES = 'custom_attributes';
     const KEY_ATTRIBUTE_CODE = \Magento\Eav\Api\Data\AttributeInterface::ATTRIBUTE_CODE;
-    const CODE_QUANTITY_AND_STOCK_STATUS = 'quantity_and_stock_status';
+    const KEY_IS_IN_STOCK = 'is_in_stock';
 
+    const CODE_QUANTITY_AND_STOCK_STATUS = 'quantity_and_stock_status';
     const PRODUCT_SKU = 'sku-test-catalog-inventory';
 
     /**
@@ -159,6 +160,36 @@ public function testSimpleProductCreationWithoutSpecifyingCatalogInventory()
         $this->assertTrue($response);
     }
 
+    /**
+     * Tests updating product stock item data when previously product was created without specified stock_item
+     */
+    public function testUpdatingQuantity()
+    {
+        // create a simple product with catalog inventory
+        $qty = null;
+        $productData = $this->getSimpleProductData($qty);
+        $response = $this->saveProduct($productData);
+        $stockItemData = $response[self::KEY_EXTENSION_ATTRIBUTES][self::KEY_STOCK_ITEM];
+
+        $this->assertEquals($qty, $stockItemData[self::KEY_QTY]);
+        $this->assertEquals(false, $stockItemData[self::KEY_IS_IN_STOCK]);
+
+        // update a created product with catalog inventory
+        $qty = 1;
+        $inStock = true;
+        $response[self::KEY_EXTENSION_ATTRIBUTES][self::KEY_STOCK_ITEM][self::KEY_QTY] = $qty;
+        $response[self::KEY_EXTENSION_ATTRIBUTES][self::KEY_STOCK_ITEM][self::KEY_IS_IN_STOCK] = $inStock;
+        $responseUpdated = $this->updateProduct($response);
+        $stockItemDataUpdated = $responseUpdated[self::KEY_EXTENSION_ATTRIBUTES][self::KEY_STOCK_ITEM];
+
+        $this->assertEquals($qty, $stockItemDataUpdated[self::KEY_QTY]);
+        $this->assertEquals($inStock, $stockItemDataUpdated[self::KEY_IS_IN_STOCK]);
+
+        // delete the product; expect that all goes well
+        $response = $this->deleteProduct($productData[ProductInterface::SKU]);
+        $this->assertTrue($response);
+    }
+
     // --- my helpers -----------------------------------------------------------------------------
 
     /**
@@ -195,7 +226,7 @@ protected function getSimpleProductData($qty = 1000)
                 [self::KEY_ATTRIBUTE_CODE => 'description', 'value' => 'My Product Description'],
                 [
                     self::KEY_ATTRIBUTE_CODE => self::CODE_QUANTITY_AND_STOCK_STATUS,
-                    'value' => ['is_in_stock' => true, 'qty' => $qty]
+                    'value' => [self::KEY_IS_IN_STOCK => true, 'qty' => $qty]
                 ],
             ];
         }
@@ -214,7 +245,7 @@ protected function getStockItemData($qty = 1000)
         return [
             self::KEY_STOCK_ITEM => [
                 self::KEY_QTY => $qty,
-                'is_in_stock' => true,
+                self::KEY_IS_IN_STOCK => true,
                 'is_qty_decimal' => false,
                 'show_default_notification_message' => false,
                 'use_config_min_qty' => true,