diff --git a/core/includes/form.inc b/core/includes/form.inc
index 260ffbe..73785c9 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -354,8 +354,9 @@ function template_preprocess_details(&$variables) {
     if (!empty($element['#attributes']['id'])) {
       $variables['summary_attributes']['aria-controls'] = $element['#attributes']['id'];
     }
-    $variables['summary_attributes']['aria-expanded'] = !empty($element['#attributes']['open']);
-    $variables['summary_attributes']['aria-pressed'] = $variables['summary_attributes']['aria-expanded'];
+    $expanded = !empty($element['#attributes']['open']) ? 'TRUE' : NULL;
+    $variables['summary_attributes']['aria-expanded'] = $expanded;
+    $variables['summary_attributes']['aria-pressed'] = $expanded;
   }
   $variables['title'] = (!empty($element['#title'])) ? $element['#title'] : '';
   $variables['description'] = (!empty($element['#description'])) ? $element['#description'] : '';