diff --git a/js/webform-admin.js b/js/webform-admin.js
index 6d5e476..2dcb914 100644
--- a/js/webform-admin.js
+++ b/js/webform-admin.js
@@ -181,6 +181,9 @@ Drupal.webform.conditionalRemove = function() {
  */
 Drupal.webform.conditionalSourceChange = function() {
   var source = $(this).val();
+  if (Drupal.settings.webform.conditionalValues.sources[source] === undefined) {
+    return;
+  }
   var dataType = Drupal.settings.webform.conditionalValues.sources[source]['data_type'];
   var $operator = $(this).parents('.webform-conditional-rule:first').find('.webform-conditional-operator select');