WebSocketSharp.WebSocketException: Not a WebSocket handshake response

when i use the client, i got an exception, like the title:

using (var ws = new WebSocket("ws://localhost:80/message/websocket?app_key=1000405"))
            {
                ws.OnMessage += (sender, e) =>
                    Console.WriteLine("Laputa says: " + e.Data);
                ws.OnClose += (send, e) =>
                {
                    Console.WriteLine("-----On close-----");
                    Console.WriteLine(e.Reason);    
                };
                ws.OnOpen += (sender, e) =>
                {
                    Console.WriteLine("----- OnOpen-----");
                    Console.WriteLine(e);
                };
                ws.OnError += (sender, e) =>
                {
                    Console.WriteLine("----- OnError-----");
                    Console.WriteLine(e);
                };
                ws.EmitOnPing = true;
                ws.Connect();
                ws.Send("BALUS");
                Console.ReadKey(true);
            }
        }

this is response:

2018/8/28 9:00:09|Fatal|WebSocket.connect:0|WebSocketSharp.WebSocketException: Not a WebSocket handshake response.
                          at WebSocketSharp.WebSocket.doHandshake () [0x0002a] in <3bfb09d1ec3f41079da19be18a3ba9a3>:0 
                          at WebSocketSharp.WebSocket.connect () [0x00083] in <3bfb09d1ec3f41079da19be18a3ba9a3>:0 
-----On close-----
An exception has occurred while connecting.

Start with the WebSocket.connect and WebSocket.doHandshake entry points named in the stack trace, using the ws://localhost:80/message/websocket?app_key=1000405 example as the reproduction. Inspect the server response to determine why it is not accepted as a WebSocket handshake; done requires identifying a reproducible cause and documenting or testing the resulting fix.