Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.299
diff -u -p -r1.299 form.inc
--- includes/form.inc	30 Oct 2008 02:35:54 -0000	1.299
+++ includes/form.inc	2 Nov 2008 00:27:33 -0000
@@ -1511,7 +1511,7 @@ function theme_fieldset($element) {
     }
 
     $element['#attributes']['class'] .= ' collapsible';
-    if ($element['#collapsed']) {
+    if (!empty($element['#collapsed'])) {
       $element['#attributes']['class'] .= ' collapsed';
     }
   }