diff --git a/site/controllers/locks_controller.php b/site/controllers/locks_controller.php
index 81115be..c42420b 100644
--- a/site/controllers/locks_controller.php
+++ b/site/controllers/locks_controller.php
@@ -55,7 +55,7 @@ class LocksController extends AppController {
   function gridDataFields(&$params, &$model) {
     $fields = parent::gridDataFields($params, $model);
     $fields[] = 'COUNT(LocksUnit.id) AS inuse';
-    $fields[] = 'Lock.qty - COUNT(LocksUnit.id) AS avail';
+    $fields[] = 'IF(Lock.qty > COUNT(LocksUnit.id), Lock.qty - COUNT(LocksUnit.id), 0) AS avail';
     return $fields;
   }