""Operation is not supported on this platform." MacOS

My Application:

using WebSocketSharp;
using System;

namespace TestProject{
    class Program{
        static void Main(string[] args){
            WebSocket ws = new WebSocket("ws://localhost:9090/ws");
            ws.SslConfiguration.EnabledSslProtocols = System.Security.Authentication.SslProtocols.Tls12;
            ws.OnMessage += (sender, e) => {
                Console.WriteLine("Says:" + e.Data);
            };
            ws.ConnectAsync();
            ws.Send("Auth");
            Console.ReadKey( true );
        }
    }
}

Error:

System.PlatformNotSupportedException: "Operation is not supported on this platform."
  at System.Func`1.BeginInvoke(AsyncCallback callback, Object object)\n   at WebSocketSharp.WebSocket.ConnectAsync()\n   at TestProject.Program.Main(String[] args) in /Users/qrpike/Projects/TestProject/TestProject/Program.cs:15

System Details:

Visual Studio Community 2017 for Mac
Version 7.6.3 (build 1)
Installation UUID: 93217eed-bb8a-4b16-9e34-95aba35b01be
Runtime:
	Mono 5.12.0.301 (2018-02/4fe3280bba1) (64-bit)
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 4.4.1.178 (master / eeaeb7e6)

	Package version: 512000301

NuGet
Version: 4.3.1.4445

.NET Core
Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.1.2
	2.0.5
SDK: /usr/local/share/dotnet/sdk/2.1.302/Sdks
SDK Versions:
	2.1.302
	2.1.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/mono/msbuild/15.0/bin/Sdks

Xamarin.Profiler
Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

Apple Developer Tools
Xcode 9.4.1 (14161)
Build 9F2000

Operating System
Mac OS X 10.13.6
Darwin 17.7.0 Darwin Kernel Version 17.7.0
    Thu Jun 21 22:53:14 PDT 2018
    root:xnu-4570.71.2~1/RELEASE_X86_64 x86_64

Start with WebSocket.ConnectAsync() and the reported Func.BeginInvoke stack trace. Reproduce the example on the listed Mono/macOS environment and determine whether the connection failure is specific to that platform. Done means the cause and a supported resolution or limitation are established and covered by an appropriate regression test or documentation.