Inconsistent behaviour for NetMQ implementation of Shared Queue (DEALER and ROUTER Sockets) from ZeroMQ Examples

I'm trying to implement with NetMQ the Shared Queue (DEALER and ROUTER Sockets) from ZeroMQ Examples. The code:

using NetMQ;using NetMQ.Sockets;using System;using System.Threading;namespace ZeroMQ.Examples{    static partial class Program    {        public static void RequestReplyClient(string[] args)        {            using (RequestSocket requester = new RequestSocket(">tcp://127.0.0.1:5559"))            {                for (int requestNumber = 0; requestNumber < 10; requestNumber++)                {                    requester.SendFrame("Hello");                    Thread.Sleep(6000);                    string requestText = requester.ReceiveFrameString();                    Console.WriteLine($"received response {requestNumber} [{requestText}]");                }            }            NetMQConfig.Cleanup();        }    }}using NetMQ;using NetMQ.Sockets;using System;using System.Threading;namespace ZeroMQ.Examples{    static partial class Program    {        public static void RequestReplyWorker(string[] args)        {            using (ResponseSocket responder = new ResponseSocket(">tcp://127.0.0.1:5560"))            {                while (true)                {                    string responseText = responder.ReceiveFrameString();                    Console.WriteLine($"received request: [{responseText}]");                    Thread.Sleep(3000);                    responder.SendFrame("world");                }            }            NetMQConfig.Cleanup();        }    }}using NetMQ;using NetMQ.Sockets;using System;using System.Threading.Tasks;namespace ZeroMQ.Examples{    static partial class Program    {        public static void RequestReplyBroker(string[] args)        {            using (RouterSocket frontend = new RouterSocket("@tcp://*:5559"))            using (DealerSocket backend = new DealerSocket("@tcp://*:5560"))            {                using (NetMQPoller items = new NetMQPoller())                {                    items.Add(frontend);                    items.Add(backend);                    frontend.ReceiveReady += (s, a) =>                    {                        string message = default(string);                        bool more = false;                        while (a.Socket.TryReceiveFrameString(TimeSpan.Zero, out message))                        {                            more = a.Socket.HasIn;                            backend.SendFrame(message, more);                            Console.WriteLine($"{message} {more}");                            if (!more) break;                        }                    };                    backend.ReceiveReady += (s, a) =>                    {                        string message = default(string);                        bool more = false;                        while (a.Socket.TryReceiveFrameString(TimeSpan.Zero, out message))                        {                            more = a.Socket.HasIn;                            frontend.SendFrame(message, more);                            Console.WriteLine($"{message} {more}");                            if (!more) break;                        }                    };                    Task.Factory.StartNew(items.Run);                    Console.WriteLine("press any key to quit ...");                    Console.ReadKey(true);                    items.Stop();                }            }            NetMQConfig.Cleanup();        }    }}

Most of the time if breaks. But once in lets say 30 times it works as expected and I don't know what I do different.

I guess there is some kind of synchronization issue. The steps that works before freezing are the following:

1. the RequestSocket from the client sends "hello" to RouterSocket from broker
2. the message is then forwarded from RouterSocker to DealerSocker
3. the message is then received by ResponseSocket from worker which replies with "word" to DealerSocket from broker
4. the message is the forwarded from DealerSocket to RouterSocket
5. everything freeze