diff --git a/src/xsprintf/xsprintf.php b/src/xsprintf/xsprintf.php
--- a/src/xsprintf/xsprintf.php
+++ b/src/xsprintf/xsprintf.php
@@ -68,7 +68,7 @@
 
         call_user_func_array(
           $callback,
-          array($userdata, $pattern, $pos, $argv[$arg], $len));
+          array($userdata, &$pattern, &$pos, &$argv[$arg], &$len));
       }
     }