diff --git a/includes/form.inc b/includes/form.inc
index 3f83b8e..1386475 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -2633,6 +2633,13 @@ function form_get_options($element, $key) {
  */
 function theme_fieldset($variables) {
   $element = $variables['element'];
+  if (!empty($element['#collapsible'])) {
+    drupal_add_library('system', 'drupal.collapse');
+    $element['#attributes']['class'][] = 'collapsible';
+    if (!empty($element['#collapsed'])) {
+      $element['#attributes']['class'][] = 'collapsed';
+    }
+  }
   element_set_attributes($element, array('id'));
   _form_set_class($element, array('form-wrapper'));