diff --git a/library/HTMLPurifier/DefinitionCache/Serializer.php b/library/HTMLPurifier/DefinitionCache/Serializer.php
index ab5398bf..26471433 100644
--- a/library/HTMLPurifier/DefinitionCache/Serializer.php
+++ b/library/HTMLPurifier/DefinitionCache/Serializer.php
@@ -54,7 +54,7 @@ class HTMLPurifier_DefinitionCache_Serializer extends
         }
         $fh = fopen($file, 'w');
         if (!$fh) return false;
-        $status = fwrite($fh, $contents);
+        $status = fwrite($fh, $data);
         fclose($fh);
         return $status;
     }