Index: modules/forum/forum.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/forum/forum.module,v
retrieving revision 1.578
diff -u -p -r1.578 forum.module
--- modules/forum/forum.module	3 Oct 2010 00:41:14 -0000	1.578
+++ modules/forum/forum.module	5 Oct 2010 23:53:34 -0000
@@ -611,7 +611,7 @@ function forum_form_alter(&$form, $form_
       // ID from the URL (e.g., if we are on a page like node/add/forum/2, we
       // expect "2" to be the ID of the forum that was requested).
       $requested_forum_id = arg(3);
-      $form['taxonomy_forums'][$langcode]['#default_value'] = is_numeric($requested_forum_id) ? $requested_forum_id : NULL;
+      $form['taxonomy_forums'][$langcode]['#default_value'] = is_numeric($requested_forum_id) ? $requested_forum_id : '';
     }
   }
 }