diff --git a/src/xsprintf/xsprintf.php b/src/xsprintf/xsprintf.php
--- a/src/xsprintf/xsprintf.php
+++ b/src/xsprintf/xsprintf.php
@@ -66,7 +66,9 @@
               'xsprintf()'));
         }
 
-        $callback($userdata, $pattern, $pos, $argv[$arg], $len);
+        if ($callback !== null) {
+          $callback($userdata, $pattern, $pos, $argv[$arg], $len);
+        }
       }
     }