diff --git a/packages/message-box/src/main.js b/packages/message-box/src/main.js
index 6a3067169..bf984abe3 100644
--- a/packages/message-box/src/main.js
+++ b/packages/message-box/src/main.js
@@ -21,6 +21,7 @@ const defaults = {
   cancelButtonText: '',
   confirmButtonClass: '',
   cancelButtonClass: '',
+  customClass: '',
   beforeClose: null
 };