What are the valid subprotograms for HttpListenerContext.AcceptWebSocketAsync?

And what is a sub-protocol, for that matter?

http://msdn.microsoft.com/en-us/library/system.net.httplistenercontext.acceptwebsocketasync(v=vs.110).aspx

I found out that you can simply set this value to nulland accept it as a valid argument. But I still could not get my application to work with this ... so I wonder if this is the right thing to do.