diff --git a/navigation404.module b/navigation404.module
index 8da3ab8..515073f 100644
--- a/navigation404.module
+++ b/navigation404.module
@@ -39,7 +39,7 @@ function navigation404_form_system_site_information_settings_alter(&$form, $form
  */
 function navigation404_site_404_submit($form, &$form_state) {
   // Make sure site_404 is not empty.
-  if ($form_state['values']['site_404'] == '') {
+  if (isset($form_state['values']['site_404']) && $form_state['values']['site_404'] == '') {
     $form_state['values']['site_404'] = NAVIGATION404_PAGE;
   }
 }