diff -u b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
--- b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php
@@ -18,13 +18,6 @@
 class LibraryDiscoveryCollector extends CacheCollector {
 
   /**
-   * The cache key.
-   *
-   * @var string
-   */
-  protected $cid;
-
-  /**
    * The cache backend.
    *
    * @var \Drupal\Core\Cache\CacheBackendInterface
@@ -66,8 +59,8 @@
    */
   public function __construct(CacheBackendInterface $cache, LockBackendInterface $lock, LibraryDiscoveryParser $discovery_parser, ThemeManagerInterface $theme_manager) {
     $this->themeManager = $theme_manager;
-    $this->cid = 'library_info:' . $this->themeManager->getActiveTheme()->getName();
-    parent::__construct($this->cid, $cache, $lock, ['library_info']);
+    $cid = 'library_info:' . $this->themeManager->getActiveTheme()->getName();
+    parent::__construct($cid, $cache, $lock, ['library_info']);
 
     $this->discoveryParser = $discovery_parser;
   }