diff --git a/build/media_src/system/js/core.es6.js b/build/media_src/system/js/core.es6.js
index 541803398cd00..0e013b577c814 100644
--- a/build/media_src/system/js/core.es6.js
+++ b/build/media_src/system/js/core.es6.js
@@ -455,7 +455,7 @@ Joomla.Modal = {
           alertClass = 'info';
         }
 
-        messagesBox.className = `alert ${alertClass}`;
+        messagesBox.className = `alert alert-${alertClass}`;
 
         // Close button
         const buttonWrapper = document.createElement('button');