diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index d5b3317..35ab1d4 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -162,16 +162,9 @@ public function disable(array $theme_list) {
     // Don't disable the default or admin themes.
     $default_theme = \Drupal::config('system.theme')->get('default');
     $admin_theme = \Drupal::config('system.theme')->get('admin');
-    $required_themes = array($default_theme, $admin_theme);
-
-    foreach ($required_themes as $theme) {
-      $pos = array_search($theme, $theme_list);
-      if ($pos !== FALSE) {
-        unset($theme_list[$pos]);
-        if (empty($theme_list)) {
-          return;
-        }
-      }
+    $theme_list = array_diff($theme_list, array($default_theme, $admin_theme));
+    if (empty($theme_list)) {
+      return;
     }
 
     $this->clearCssCache();