diff --git a/socket.io/socket.io.d.ts b/socket.io/socket.io.d.ts
index a34a78b8c..355606858 100644
--- a/socket.io/socket.io.d.ts
+++ b/socket.io/socket.io.d.ts
@@ -647,7 +647,7 @@ declare module SocketIO {
 		 * @param close If true, also closes the underlying connection
 		 * @return This Socket
 		 */
-		disconnect( close: boolean ): Socket;
+		disconnect( close?: boolean ): Socket;
 		
 		/**
 		 * Adds a listener for a particular event. Calling multiple times will add