--- drupal.js
+++ drupal.js
@@ -28,6 +28,7 @@
  */
 Drupal.redirectFormButton = function (uri, button, handler) {
   // Trap the button
+  if (button === undefined) return true;
   button.onmouseover = button.onfocus = function() {
     button.onclick = function() {
       // Create target iframe