diff --git a/core/includes/form.inc b/core/includes/form.inc
index 5634be8..276ed30 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -2795,10 +2795,15 @@ function theme_fieldset($variables) {
element_set_attributes($element, array('id'));
_form_set_attributes($element, array('form-wrapper'));
+ $legend_attributes = array();
+ if ($element['#title_display'] == 'invisible') {
+ $legend_attributes['class'][] = 'element-invisible';
+ }
+
$output = '