diff --git a/lib/util.php b/lib/util.php
index 221175ccd2..ca9006d64e 100644
--- a/lib/util.php
+++ b/lib/util.php
@@ -467,7 +467,7 @@ function common_replace_urls_callback($text, $callback) {
         $url = (mb_strpos($orig_url, htmlspecialchars($url)) === FALSE) ? $url:htmlspecialchars($url);
 
         // Call user specified func
-        $modified_url = $callback($url);
+        $modified_url = call_user_func($callback, $url);
 
         // Replace it!
         $start = mb_strpos($text, $url, $offset);