WEB Sockets server C#

I am learning websockets and I have made a websocket service, but in the client it says : 

"WebSocket connection to 'ws://localhost:7784/reverse' failed: Error during WebSocket handshake: Unexpected response code: 501"

My code is :

using Microsoft.ServiceModel.WebSockets;
using System;
using System.Linq;

namespace wsServerr
{
    public class Reverse : WebSocketsService
    {
        public override void OnMessage(string value)
        {
            string reversed = string.Join("", value.Reverse());
            SendMessage(reversed);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var host = new WebSocketsHost<Reverse>();
            host.AddWebSocketsEndpoint("ws://localhost:7784/reverse");
            host.Open();
            Console.ReadLine();
        }
    }
}