diff --git a/core/modules/ckeditor5/ckeditor5.module b/core/modules/ckeditor5/ckeditor5.module
index d74926fa72..1b121e2805 100644
--- a/core/modules/ckeditor5/ckeditor5.module
+++ b/core/modules/ckeditor5/ckeditor5.module
@@ -358,8 +358,9 @@ function ckeditor5_library_info_alter(&$libraries, $extension) {
 
     $themes = [];
     $default_theme = \Drupal::configFactory()->get('system.theme')->get('default');
-    $default_theme_info = \Drupal::service('theme_handler')->listInfo()[$default_theme]->info;
-    $themes[$default_theme] = $default_theme_info;
+    if (!empty($default_theme)) {
+      $themes[$default_theme] = \Drupal::service('theme_handler')->listInfo()[$default_theme]->info;
+    }
 
     $admin_theme = \Drupal::configFactory()->get('system.theme')->get('admin');
     if (!empty($admin_theme) && $admin_theme !== $default_theme) {
diff --git a/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php
index 3ccd17841e..f55181dd61 100644
--- a/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditorStylesheetsWarningTest.php
@@ -34,10 +34,12 @@ public function testWarningFilterUI() {
     $assert_session = $this->assertSession();
     $this->addNewTextFormat($page, $assert_session);
     $this->drupalGet('admin/config/content/formats/manage/ckeditor5');
-    $this->assertFalse($assert_session->waitForText('Your default theme "test_ckeditor_stylesheets_without_5" has ckeditor_stylesheets configured, but no corresponding ckeditor5-stylesheets configuration. See https://www.drupal.org/node/3259165 for details.', 5000));
+
+    $warning = 'Your default theme test_ckeditor_stylesheets_without_5 has ckeditor_stylesheets configured, but no corresponding ckeditor5-stylesheets configuration. See https://www.drupal.org/node/3259165 for details';
+    $this->assertFalse($assert_session->waitForText($warning, 5000));
     $this->installThemeThatTriggersWarning();
     $this->drupalGet('admin/config/content/formats/manage/ckeditor5');
-    $this->assertTrue($assert_session->waitForText('Your default theme "test_ckeditor_stylesheets_without_5" has ckeditor_stylesheets configured, but no corresponding ckeditor5-stylesheets configuration. See https://www.drupal.org/node/3259165 for details.'));
+    $this->assertTrue($assert_session->waitForText($warning));
   }
 
 }