diff --git a/framework/core/js/forum/src/components/Composer.js b/framework/core/js/forum/src/components/Composer.js
index 05d50234e..0bcdb72d9 100644
--- a/framework/core/js/forum/src/components/Composer.js
+++ b/framework/core/js/forum/src/components/Composer.js
@@ -124,7 +124,7 @@ class Composer extends Component {
     // component a chance to scream at the user to make sure they don't
     // unintentionally lose any contnet.
     window.onbeforeunload = () => {
-      return (this.component && this.component.preventExit()) || null;
+      return (this.component && this.component.preventExit()) || undefined;
     };
 
     const handlers = {};