updated drupal core to 7.51

includes/ajax.inc

@@ -394,7 +394,7 @@ function ajax_form_callback() {
   if (!empty($form_state['triggering_element'])) {
     $callback = $form_state['triggering_element']['#ajax']['callback'];
   }
-  if (!empty($callback) && function_exists($callback)) {
+  if (!empty($callback) && is_callable($callback)) {
     $result = $callback($form, $form_state);
 
     if (!(is_array($result) && isset($result['#type']) && $result['#type'] == 'ajax')) {