diff --git a/index.bs b/index.bs
index 53136ba4..19e5862b 100644
--- a/index.bs
+++ b/index.bs
@@ -5392,7 +5392,7 @@ interfaces will always return 0.
     network protocol "protocol X", could look something like this:
 
     <pre highlight=webidl>
-        [Exposed=Window]
+        [Exposed=Window, Serializable]
         interface ProtocolXError : DOMException {
           constructor(optional DOMString message = "", ProtocolXErrorOptions options);