diff --git a/entityreference_prepopulate.module b/entityreference_prepopulate.module
index 9c06140..9a51668 100644
--- a/entityreference_prepopulate.module
+++ b/entityreference_prepopulate.module
@@ -159,6 +159,8 @@ function entityreference_prepopulate_field_access($op, $field, $entity_type, $en
   }
 
   $settings = $instance['settings']['behaviors']['prepopulate'];
+  // ensure that needed keys are defined
+  $settings += array('action' => NULL, 'action_on_edit' => NULL, 'fallback' => NULL);
 
   if (!empty($settings['skip_perm']) && user_access($settings['skip_perm'])) {
     return;