diff --git a/core/includes/form.inc b/core/includes/form.inc
index efbac99..b967b32 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -251,7 +251,7 @@ function template_preprocess_details(&$variables) {
       $variables['summary_attributes']['aria-controls'] = $element['#attributes']['id'];
     }
     $variables['summary_attributes']['aria-expanded'] = !empty($element['#attributes']['open']) ? 'true' : 'false';
-    $variables['summary_attributes']['aria-pressed'] = $variables['summary_attributes']['aria-expanded'];
+    $variables['summary_attributes']['aria-pressed'] = !empty($element['#attributes']['open']) ? 'true' : 'false';
   }
   $variables['title'] = (!empty($element['#title'])) ? $element['#title'] : '';
   $variables['description'] = (!empty($element['#description'])) ? $element['#description'] : '';