diff --git a/core/modules/system/src/Tests/Ajax/FrameworkTest.php b/core/modules/system/src/Tests/Ajax/FrameworkTest.php
index c7ccd8b..7b78b5c 100644
--- a/core/modules/system/src/Tests/Ajax/FrameworkTest.php
+++ b/core/modules/system/src/Tests/Ajax/FrameworkTest.php
@@ -216,4 +216,14 @@ public function testLazyLoadOverriddenCSS() {
     // cache control.
     $this->assertNoText('system.module.css?', 'Ajax lazy loading does not add overridden CSS files.');
   }
+
+  /**
+   * Tests if the 'theme_token' key of 'ajaxPageState' is computed.
+   */
+  public function testThemeToken() {
+    $this->drupalGet('ajax_forms_test_lazy_load_form');
+    $settings = $this->getDrupalSettings();
+    $this->assertTrue(is_null($settings['ajaxPageState']['theme_token']));
+  }
+
 }
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index b0420c1..314ec39 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -691,9 +691,10 @@ function system_js_settings_alter(&$settings, AttachedAssetsInterface $assets) {
       // The theme token is only validated when the theme requested is not the
       // default, so don't generate it unless necessary.
       // @see \Drupal\Core\Theme\AjaxBasePageNegotiator::determineActiveTheme()
-      if (\Drupal::theme()->getActiveTheme()->getName() !== \Drupal::service('theme_handler')->getDefault()) {
+      $active_theme_key = \Drupal::theme()->getActiveTheme()->getName();
+      if ($active_theme_key !== \Drupal::service('theme_handler')->getDefault()) {
         $settings['ajaxPageState']['theme_token'] = \Drupal::csrfToken()
-          ->get(\Drupal::theme()->getActiveTheme()->getName());
+          ->get($active_theme_key);
       }
     }
   }