issues/36800 - Can't create credit memo with free shipping

Author: yogesh-valiya

app/code/Magento/Sales/Model/Order/Creditmemo/Total/Tax.php

@@ -134,8 +134,8 @@ public function collect(Creditmemo $creditmemo)
             $baseShippingDiscountTaxCompensationAmount = 0;
             $shippingDelta = $baseOrderShippingAmount - $baseOrderShippingRefundedAmount;
 
-            if ($shippingDelta > $creditmemo->getBaseShippingAmount() ||
-                $this->isShippingIncludeTaxWithTaxAfterDiscount($order->getStoreId())) {
+            if ($orderShippingAmount > 0 && ($shippingDelta > $creditmemo->getBaseShippingAmount() ||
+                $this->isShippingIncludeTaxWithTaxAfterDiscount($order->getStoreId()))) {
                 $part = $creditmemo->getShippingAmount() / $orderShippingAmount;
                 $basePart = $creditmemo->getBaseShippingAmount() / $baseOrderShippingAmount;
                 $shippingTaxAmount = $order->getShippingTaxAmount() * $part;