diff --git a/src/Controller/APIController.php b/src/Controller/APIController.php
index 10a5d26e..a6ea86cf 100644
--- a/src/Controller/APIController.php
+++ b/src/Controller/APIController.php
@@ -718,11 +718,11 @@ class APIController extends Controller implements LoggedController
         $cvs = $cust->getVehicles();
         foreach ($cvs as $cv)
         {
-            $battery_id = '';
+            $battery_id = null;
             if ($cv->getCurrentBattery() != null)
                 $battery_id = $cv->getCurrentBattery()->getID();
 
-            $wty_ex = '';
+            $wty_ex = null;
             if ($cv->getWarrantyExpiration() != null)
                 $wty_ex = $cv->getWarrantyExpiration()->format('Y-m-d');