diff --git a/index.html b/index.html
index 2fab555..4a537bc 100644
--- a/index.html
+++ b/index.html
@@ -436,8 +436,8 @@ <h2>
       attribute EventHandler onconnect;
       attribute EventHandler ondisconnect;
       readonly attribute boolean connected;
-      readonly attribute ReadableStream readable;
-      readonly attribute WritableStream writable;
+      readonly attribute ReadableStream? readable;
+      readonly attribute WritableStream? writable;
 
       SerialPortInfo getInfo();