diff --git a/include/utils/VTCacheUtils.php b/include/utils/VTCacheUtils.php
index 1134965ef0e07ccf5efc074a79b2eec284b09d77..3a286e03c386935e05e85e0040674252f4086dd7 100644
--- a/include/utils/VTCacheUtils.php
+++ b/include/utils/VTCacheUtils.php
@@ -428,7 +428,7 @@ class VTCacheUtils {
         self::$_userSignature[$userName] = $signature;
     }
     public static function getUserSignature($userName) {
-        return self::$_userSignature[$userName];
+        return isset($_userSignature[$userName]) ? self::$_userSignature[$userName] : null;
     }
 
     static $_userFullName = array();