diff --git a/core/MY_Model.php b/core/MY_Model.php
index 05add98..79beee7 100644
--- a/core/MY_Model.php
+++ b/core/MY_Model.php
@@ -694,7 +694,7 @@ public function serialize($row)
     {
         foreach ($this->callback_parameters as $column)
         {
-            $row[$column] = serialize($row[$column]);
+            if (isset($row[$column])) $row[$column] = serialize($row[$column]);
         }
 
         return $row;