diff --git a/entityreference.module b/entityreference.module
index 425c8aa..cfc67bc 100644
--- a/entityreference.module
+++ b/entityreference.module
@@ -794,11 +794,11 @@ function entityreference_field_widget_error($element, $error) {
  *   The bundle name.
  * @param $entity_id
  *   Optional; The entity ID the entity-reference field is attached to.
- *   Defaults to ''.
+ *   Defaults to 'NULL'.
  * @param $string
  *   The label of the entity to query by.
  */
-function entityreference_autocomplete_callback($type, $field_name, $entity_type, $bundle_name, $entity_id = '', $string = '') {
+function entityreference_autocomplete_callback($type, $field_name, $entity_type, $bundle_name, $entity_id = 'NULL', $string = '') {
   $field = field_info_field($field_name);
   $instance = field_info_instance($entity_type, $field_name, $bundle_name);
   $matches = array();