diff --git a/doc/events/websocket_message.md b/doc/events/websocket_message.md
index 53b9d4bd2..839e214cc 100644
--- a/doc/events/websocket_message.md
+++ b/doc/events/websocket_message.md
@@ -10,6 +10,7 @@ This event is normally handled by @{http.Websocket.receive}, but it can also be
 1. @{string}: The event name.
 2. @{string}: The URL of the WebSocket.
 3. @{string}: The contents of the message.
+4. @{boolean}: Whether this is a binary message.
 
 ## Example
 Prints a message sent by a WebSocket: