Reflection error with self-hosted nancyfx app on mono

After building and successfully running a self-hosted NancyFx application on windows with visual studio I went on and tried to run the application on linux with mono. I was able to build the solution (using xbuild) but when running the app I get a strange error:

Unhandled Exception:
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> 
System.TypeLoadException: Could not load type 'System.Net.HttpListener'     
from assembly 'System, Version=4.0.0.0, Culture=neutral,       
PublicKeyToken=b77a5c561934e089'.
at Microsoft.Owin.Host.HttpListener.OwinServerFactory.Initialize (IDictionary`2 properties) <0x413ae4f0 + 0x000eb> in <filename unknown>:0

For some reason, the app tries to initialize class HttpListener from the System assembly, but HttpListener lives in System.Netassembly. I looked in the GAC folder and found that System.Net is there as expected. I moved a copy to the bin folder where the other dependencies of the app live but that didn't solve the problem. Any help would be appreciated!!

here is my OWIN-related code:

// Startup.cs 
using Owin;

namespace MyApp
{
    public class Startup
    {
        public void Configuration(IAppBuilder app)
        {
             app.UseNancy();
        }
    }
}

// Program.cs
using System;
using Microsoft.Owin.Hosting;

namespace MyApp
{
    public class Program
    {
        static void Main(string[] args)
        {
            var url = "http://127.0.0.1:8080/";
            using (WebApp.Start<Startup>(url))
            {
                Console.WriteLine("Running on {0}", url);
                Console.WriteLine("Press enter to exit");
                Console.ReadLine();
            }
        }
    }
}