diff --git a/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
index 1cd883b..5cd2ee3 100644
--- a/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
+++ b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
@@ -73,18 +73,17 @@ public function applies(RouteMatchInterface $route_match) {
    * {@inheritdoc}
    */
   public function determineActiveTheme(RouteMatchInterface $route_match) {
-    if ($ajax_page_state = $this->requestStack->getCurrentRequest()->request->get('ajax_page_state')) {
-      $theme = $ajax_page_state['theme'];
-      $token = $ajax_page_state['theme_token'];
+    $ajax_page_state = $this->requestStack->getCurrentRequest()->request->get('ajax_page_state');
+    $theme = $ajax_page_state['theme'];
+    $token = $ajax_page_state['theme_token'];
 
-      // Prevent a request forgery from giving a person access to a theme they
-      // shouldn't be otherwise allowed to see. However, since everyone is
-      // allowed to see the default theme, token validation isn't required for
-      // that, and bypassing it allows most use-cases to work even when accessed
-      // from the page cache.
-      if ($theme === $this->configFactory->get('system.theme')->get('default') || $this->csrfGenerator->validate($token, $theme)) {
-        return $theme;
-      }
+    // Prevent a request forgery from giving a person access to a theme they
+    // shouldn't be otherwise allowed to see. However, since everyone is
+    // allowed to see the default theme, token validation isn't required for
+    // that, and bypassing it allows most use-cases to work even when accessed
+    // from the page cache.
+    if ($theme === $this->configFactory->get('system.theme')->get('default') || $this->csrfGenerator->validate($token, $theme)) {
+      return $theme;
     }
   }
 
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxThemeTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxThemeTest.php
index c536fe8..819c430 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxThemeTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxThemeTest.php
@@ -32,7 +32,7 @@ public function testAjaxWithAdminRoute() {
     $assert = $this->assertSession();
     $assert->pageTextContains('Current theme: seven');
 
-    // Now
+    // Now click the modal, which should use also use the admin theme.
     $this->drupalGet('ajax-test/dialog');
     $assert->pageTextNotContains('Current theme: stable');
     $this->clickLink('Link 8 (ajax)');