diff --git a/src/Controller/APIController.php b/src/Controller/APIController.php
index 90fe47bb..15b979c1 100644
--- a/src/Controller/APIController.php
+++ b/src/Controller/APIController.php
@@ -2826,6 +2826,9 @@ class APIController extends Controller implements LoggedController
                 'is_registered' => $is_registered,
                 'customer' => $customer,
                 'battery' => $battery,
+                'message' => [
+                    'register_error' => 'Warranty serial code has already been registered.',
+                ],
             ];
         }