zeromq/libzmq

I've created such server

public static void Main(string[] args)
        {
            using (var server = new ResponseSocket("@tcp://localhost:5555")) // bind
            {
                while (true)
                {
                    // Receive the message from the server socket
                    string m1 = server.ReceiveFrameString();
                    Console.WriteLine("From Client: {0}", m1);

                    // Send a response back from the server
                    server.SendFrame("Hi Back");
                }
            }
        }

and such client

public static void Main(string[] args)
        {
            using (var client = new RequestSocket(">tcp://localhost:5555"))  // connect
            {
                while (true)
                {
                    // Send a message from the client socket
                    if (client.TrySendFrame(TimeSpan.FromSeconds(5), "Hello"))
                    {
                        // Receive the response from the client socket
                        string m2 = client.ReceiveFrameString();
                        Console.WriteLine("From Server: {0}", m2);
                    }
                    else Console.WriteLine("Data is not sent");
                    Thread.Sleep(1000);
                }
            }
        }

I've started client and then server. All worked but! when I stoped server and client via Ctrl+C after restarting worked nothing. And when I change port to 5556 on client and server then all began to work. I guess that some handled did not close.
What is your opinion? Is it good?