diff --git a/interfaces/html.idl b/interfaces/html.idl
index 486875ec274f84c..5a77c9b31055f1c 100644
--- a/interfaces/html.idl
+++ b/interfaces/html.idl
@@ -1166,9 +1166,10 @@ dictionary SubmitEventInit : EventInit {
   HTMLElement? submitter = null;
 };
 
-[Exposed=Window,
- Constructor(DOMString type, optional FormDataEventInit eventInitDict = {})]
+[Exposed=Window]
 interface FormDataEvent : Event {
+  constructor(DOMString type, FormDataEventInit eventInitDict);
+
   readonly attribute FormData formData;
 };