diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index dc5f5c6..14fb775 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2052,7 +2052,7 @@ function template_preprocess_maintenance_page(&$variables) {
   $page_object = $variables['page']['#page'];
   $attributes = $page_object->getBodyAttributes();
   $classes = $attributes['class'];
-  $classes = 'maintenance-page';
+  $classes[] = 'maintenance-page';
   if (isset($variables['db_is_active']) && !$variables['db_is_active']) {
     $classes[] = 'db-offline';
   }