C# Lidgren network MissingMethodException

I'm getting the following error

 MissingMethodException: Method not found: 'System.Threading.Monitor.Enter'.
 Lidgren.Network.NetPeer.Start ()

from this code:

 using UnityEngine;
 
 using System;
 using System.Collections;
 
 using Lidgren.Network;
 
 public class NetworkConnection {
     public static NetClient client;
     public static NetPeerConfiguration config;
     
     static NetworkConnection() {
         config    = new NetPeerConfiguration("arena");
         client    = new NetClient(config);
         
         client.Start();
         client.Connect("127.0.0.1", 3054);
     }
 }

My feeble attempts at googling the error haven't told me what's wrong so I thought to ask here. I can't see anything that differs from code in samples of the Lidgren network library so I'm stumped.