Index: modules/field_ui/field_ui.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/field_ui/field_ui.admin.inc,v
retrieving revision 1.26
diff -u -p -r1.26 field_ui.admin.inc
--- modules/field_ui/field_ui.admin.inc	29 Oct 2009 07:21:04 -0000	1.26
+++ modules/field_ui/field_ui.admin.inc	2 Nov 2009 22:57:57 -0000
@@ -1305,6 +1305,7 @@ function field_ui_field_edit_form_submit
   if (isset($instance_values['default_value_widget'])) {
     $langcode = $form['instance']['default_value_widget'][$field_name]['#language'];
     $instance_values['default_value'] = $instance_values['default_value_widget'][$field_name][$langcode];
+    field_set_empty($field, &$instance_values['default_value']); //unset if empty 
     unset($instance_values['default_value_widget']);
   }