diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
index 09cb0ec..d3a0dce 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
@@ -170,7 +170,7 @@ public function form(array $form, array &$form_state) {
       '#description' => $this->t('Briefly desribe the changes you have made.'),
     );
 
-    $form['#attached']['library'][] = array('custom_block', 'drupal.custom_block');
+    $form['#attached']['library'][] = 'custom_block/drupal.custom_block';
 
     return parent::form($form, $form_state, $block);
   }