diff --git a/DataStorage/Database/DataMapperAbstract.php b/DataStorage/Database/DataMapperAbstract.php
index 2a322af40..594330679 100644
--- a/DataStorage/Database/DataMapperAbstract.php
+++ b/DataStorage/Database/DataMapperAbstract.php
@@ -347,7 +347,7 @@ class DataMapperAbstract implements DataMapperInterface
         self::extend(__CLASS__);
 
         if ($obj === null ||
-            (is_object($obj) && strpos($className = $obj, '\Null') !== false)
+            (is_object($obj) && strpos($className = get_class($obj), '\Null') !== false)
         ) {
             return null;
         }