[Lidgren network] Method not found compiler error

I am trying to create my client with the imported lidgren network. When i just try to do a simple connect to the server i will get a compiler error telling me that there is a method missing. I dont know how to go forward from this. Do you guys know what the problem is and how i could fix this.

My Code: using UnityEngine; using System.Collections; using System.Net; using System.Threading; using System.IO; using Lidgren.Network;

 public class NetworkHandler : MonoBehaviour 
 {
     NetPeerConfiguration config;
     NetClient client;
 
     void Start () 
     {
         config = new NetPeerConfiguration("castaways");
         config.Port = 6112;
         config.EnableMessageType(NetIncomingMessageType.DiscoveryRequest);
         client = new NetClient(config);
 
         StartClient();
         Connect("localhost",6112);
 
     }
 
 
     public void Connect(string host, int port)
     {
         client.Connect(host,port);
     }
 
     public void StartClient()
     {
         Debug.Log("Starting Client");
         client.Start();
     }
 
     public void ShutDownClient()
     {
         Debug.Log("Client shutdown");
         client.Shutdown("Goodbye");
     }
 
 
     void Update () 
     {
     
     }
 }

I attached this script to a empty gameObject and i got the following error from it

MissingMethodException: Method not found: 'System.Threading.Monitor.Enter'. Lidgren.Network.NetPeer.Start () NetworkHandler.StartClient () (at Assets/Scripts/NetworkHandler.cs:34) NetworkHandler.Start () (at Assets/Scripts/NetworkHandler.cs:20)

Please help //Cheers