diff --git a/core/lib/Drupal/Core/Theme/ActiveTheme.php b/core/lib/Drupal/Core/Theme/ActiveTheme.php
index e94fbd77cce9c0acc62cccca70e7b4e1635ae232..bb5c766b04f7e16590738084f7f7d10a061f3152 100644
--- a/core/lib/Drupal/Core/Theme/ActiveTheme.php
+++ b/core/lib/Drupal/Core/Theme/ActiveTheme.php
@@ -82,6 +82,13 @@ class ActiveTheme {
    */
   protected $librariesOverride;
 
+  /**
+   * The list of libraries-extend definitions.
+   *
+   * @var array
+   */
+  protected $librariesExtend;
+
   /**
    * Constructs an ActiveTheme object.
    *