https://github.com/magento/magento2/issues/35440

--- a/vendor/magento/module-quote/Model/QuoteManagement.php
+++ b/vendor/magento/module-quote/Model/QuoteManagement.php
@@ -425,6 +425,7 @@
             }
             $quote->setCustomerIsGuest(true);
             $groupId = $customer ? $customer->getGroupId() : GroupInterface::NOT_LOGGED_IN_ID;
+            $groupId = $groupId ?: GroupInterface::NOT_LOGGED_IN_ID;
             $quote->setCustomerGroupId($groupId);
         }