MAGETWO-80371: Exception when product added to cart with tiered price reduced to $0.00

Author: AVoskoboinikov

app/code/Magento/Catalog/Test/Unit/Model/Product/Attribute/Backend/TierpriceTest.php

@@ -141,6 +141,10 @@ public function testSetPriceData()
     {
         $attributeName = 'tier_price';
         $tierPrices = [
+            [
+                'price' => 0,
+                'all_groups' => 1,
+            ],
             [
                 'price' => 10,
                 'all_groups' => 1,
@@ -153,6 +157,12 @@ public function testSetPriceData()
         $productPrice = 20;
         $allCustomersGroupId = 32000;
         $finalTierPrices = [
+            [
+                'price' => 0,
+                'all_groups' => 1,
+                'website_price' => 0,
+                'cust_group' => 32000,
+            ],
             [
                 'price' => 10,
                 'all_groups' => 1,
@@ -170,8 +180,8 @@ public function testSetPriceData()
             ->disableOriginalConstructor()->getMock();
         $allCustomersGroup = $this->getMockBuilder(\Magento\Customer\Api\Data\GroupInterface::class)
             ->disableOriginalConstructor()->getMock();
-        $this->groupManagement->expects($this->once())->method('getAllCustomersGroup')->willReturn($allCustomersGroup);
-        $allCustomersGroup->expects($this->once())->method('getId')->willReturn($allCustomersGroupId);
+        $this->groupManagement->expects($this->exactly(2))->method('getAllCustomersGroup')->willReturn($allCustomersGroup);
+        $allCustomersGroup->expects($this->exactly(2))->method('getId')->willReturn($allCustomersGroupId);
         $object->expects($this->once())->method('getPrice')->willReturn($productPrice);
         $this->attribute->expects($this->atLeastOnce())->method('isScopeGlobal')->willReturn(true);
         $object->expects($this->once())->method('getStoreId')->willReturn(null);

tier_price.patch

@@ -0,0 +1,13 @@
+diff --git a/app/code/Magento/Catalog/Model/Product/Attribute/Backend/GroupPrice/AbstractGroupPrice.php b/app/code/Magento/Catalog/Model/Product/Attribute/Backend/GroupPrice/AbstractGroupPrice.php
+index f7ab3f5..cd686c0 100644
+--- a/app/code/Magento/Catalog/Model/Product/Attribute/Backend/GroupPrice/AbstractGroupPrice.php
++++ b/app/code/Magento/Catalog/Model/Product/Attribute/Backend/GroupPrice/AbstractGroupPrice.php
+@@ -360,7 +360,7 @@ abstract class AbstractGroupPrice extends Price
+     {
+         /** @var array $priceItem */
+         foreach ($data as $key => $priceItem) {
+-            if (isset($priceItem['price']) && $priceItem['price'] > 0) {
++            if (array_key_exists('price', $priceItem)) {
+                 $data[$key]['website_price'] = $priceItem['price'];
+             }
+             if ($priceItem['all_groups']) {