diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTestBase.php b/core/modules/block/lib/Drupal/block/Tests/BlockTestBase.php
index 96ea288..1ebdd48 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTestBase.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTestBase.php
@@ -68,8 +68,7 @@ function setUp() {
     );
 
     $default_theme = variable_get('theme_default', 'stark');
-    $manager = $this->container->get('plugin.manager.block');
-    $instances = config_get_storage_names_with_prefix('plugin.core.block.' . $default_theme);
+    $instances = config_get_storage_names_with_prefix('block.block.' . $default_theme);
     foreach ($instances as $plugin_id) {
       \Drupal::config($plugin_id)->delete();
     }