MC-36838: Cannot checkout with automatic customer group assignment

Author: cpartica

app/code/Magento/Quote/Observer/Frontend/Quote/Address/CollectTotalsObserver.php

@@ -135,8 +135,10 @@ public function execute(\Magento\Framework\Event\Observer $observer)
             $address->setPrevQuoteCustomerGroupId($quote->getCustomerGroupId());
             $quote->setCustomerGroupId($groupId);
             $this->customerSession->setCustomerGroupId($groupId);
-            $customer->setGroupId($groupId);
-            $quote->setCustomer($customer);
+            if ($customer->getId() !== null) {
+                $customer->setGroupId($groupId);
+                $quote->setCustomer($customer);
+            }
         }
     }
 }