diff -u b/date_popup/date_popup.module b/date_popup/date_popup.module
--- b/date_popup/date_popup.module
+++ b/date_popup/date_popup.module
@@ -342,9 +342,12 @@
     'yearRange' => $year_range,
     // Custom setting, will be expanded in Drupal.behaviors.date_popup()
     'fromTo' => isset($fromto),
-    'syncEndDate' => $element['#instance']['settings']['default_value2'] == 'sync',
   );
 
+  if (!empty($element['#instance'])) {
+    $settings['syncEndDate'] = $element['#instance']['settings']['default_value2'] == 'sync';
+  }
+
   // Create a unique id for each set of custom settings.
   $id = date_popup_js_settings_id($element['#id'], 'datepicker', $settings);