module = null

I have a feature for a content type and the feature includes field instances. The feature now shows as overridden. (Probably overridden after a features module update.) The only change showed when diffing the existing feature with a recreated version is

@@ -542,7 +542,7 @@ function ct_event_field_default_field_instances() {
     'display' => array(
       'default' => array(
         'label' => 'above',
-        'module' => 'registration',
+        'module' => NULL,
         'settings' => array(
           'label' => ' ',
         ),
@@ -563,7 +563,7 @@ function ct_event_field_default_field_instances() {
       ),
       'token' => array(
         'label' => 'above',
-        'module' => 'registration',
+        'module' => NULL,
         'settings' => array(
           'label' => ' ',
         ),

The field is added to the content type by the Entity Registration module. (And the Entity Registration module is enabled on the site.)

What I don't understand is why features now wants to change the value for module from 'registration' to 'null'?

Any ideas?