Android : Xamarin Android Auth0 + Twitter4J - java.io.IOException: Received authentication challenge is null

I'm using Autho0 to authenticate users of my app. I am trying to send tweets via Twitter4J (the Monodroid wrapper from Xamarin). When I attempt to send the tweet I get the "Received authentication challenge is null" exception. Here's the code:

using System;
using System.Threading;

using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using Auth0.SDK;
using Newtonsoft.Json.Linq;
using Twitter4j;
using Twitter4j.Conf;

namespace MyNamespace
{
    [Activity (Label = "App", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        Auth0User _user = null;
        protected async override void OnCreate (Bundle bundle)
        {
            base.OnCreate (bundle);

            // Set our view from the "main" layout resource
            SetContentView (Resource.Layout.Main);

            var auth0 = new Auth0Client(
                "URL",
                "KEY");

            _user = await auth0.LoginAsync(this);

            if (_user != null) {
                var name = (string)_user.Profile["name"];
                var provider = (string)_user.Profile ["identities"] [0] ["provider"];
                var accessToken = (string)_user.Profile ["identities"] [0] ["access_token"];
                var accessTokenSecret = (string)_user.Profile ["identities"] [0] ["access_token_secret"];

                if (provider == "twitter") {
                    ConfigurationBuilder config = new ConfigurationBuilder();
                    config.SetOAuthConsumerKey("CONSUMERKEY");
                    config.SetOAuthConsumerSecret("CONSUMERSECRET");
                    config.SetOAuthAccessToken(accessToken);
                    config.SetOAuthAccessTokenSecret(accessTokenSecret);
                    config.SetUser(name);

                    var factory = new Twitter4j.TwitterFactory (config.Build());

                    //factory.Instance.
                    var twitter = factory.Instance;

                    ThreadPool.QueueUserWorkItem(state =>
                        {
                            try
                            {
                                twitter.UpdateStatus("The first tweet from our app!");
                            }
                            catch (Java.Lang.Exception ex)
                            {
                                Console.WriteLine(ex);
                            }
                        });
                }
            } else {
                System.Diagnostics.Process.GetCurrentProcess ().CloseMainWindow ();
            }
        }
    }
}