diff -u b/core/modules/block/block.module b/core/modules/block/block.module
--- b/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -270,8 +270,8 @@
   }
 
   // Proactively add aria-describedby if possible to improve accessibility.
-  if ($variables['label'] && isset($variables['attributes']['role'])) {
-    $variables['title_attributes']['id'] = drupal_html_id($variables['label']);
+  if (isset($variables['elements']['#block']->admin_label) && isset($variables['attributes']['role'])) {
+    $variables['title_attributes']['id'] = drupal_html_id($variables['elements']['#block']->admin_label);
     $variables['attributes']['aria-describedby'] = $variables['title_attributes']['id'];
   }
 
@@ -384,8 +384,8 @@
   }
 
   // Proactively add aria-describedby if possible to improve accessibility.
-  if (isset($variables['elements']['#block']->admin_label) && isset($variables['attributes']['role'])) {
-    $variables['title_attributes']['id'] = drupal_html_id($variables['elements']['#block']->admin_label);
+  if (isset($variables['elements']['#block']->subject) && isset($variables['attributes']['role'])) {
+    $variables['title_attributes']['id'] = drupal_html_id($variables['elements']['#block']->subject);
     $variables['attributes']['aria-describedby'] =  $variables['title_attributes']['id'];
   }