Isolate.Invoke.Event not working

using System.Net;
using async.tests.native.Version6;
using NUnit.Framework;
using TypeMock.ArrangeActAssert;

namespace async.tests.typemock
{
    [TestFixture, Isolated]
    public class ArtikelRepositoryTests
    {
        private ArtikelRepository sut;
        private WebClient webClient;

        [SetUp]
        public void Setup() {
            webClient = Isolate.Fake.Instance<WebClient>(Members.ReturnRecursiveFakes);
            Isolate.Swap.NextInstance<WebClient>().With(webClient);
            Isolate.WhenCalled(() => webClient.DownloadDataCompleted += null).CallOriginal();

            sut = new ArtikelRepository();
        }

        [Test, Isolated]
        public void Name() {
            var e = Isolate.Fake.Instance<DownloadDataCompletedEventArgs>();

            var count = 0;
            sut.OnArtikelGeladen += delegate { count++; };

            sut.LadeArtikel();
            Isolate.Invoke.Event(() => webClient.DownloadDataCompleted += null, null, e);

            Assert.That(count, Is.EqualTo(1));
        }
    }
}

using System;
using System.Collections.Generic;
using System.Net;
using async.tests.native.Version5;

namespace async.tests.native.Version6
{
    public class ArtikelRepository : IArtikelRepository
    {
        private readonly IArtikelReader artikelReader;

        public ArtikelRepository()
            : this(new ArtikelReader()) {
        }

        internal ArtikelRepository(IArtikelReader artikelReader) {
            this.artikelReader = artikelReader;
        }

        public void LadeArtikel() {
            var webClient = new WebClient();
            webClient.DownloadDataCompleted += OnCompleted;
            var url = new Uri("http://example.de/artikel");
            webClient.DownloadDataAsync(url);
        }

        public event Action<IEnumerable<Artikel>> OnArtikelGeladen = delegate { };

        private void OnCompleted(object sender, DownloadDataCompletedEventArgs e) {
            var artikel = artikelReader.Read(e.Result);
            OnArtikelGeladen(artikel);
        }
    }
}

using System.Collections.Generic;

namespace async.tests.native.Version5
{
    public class ArtikelReader : IArtikelReader
    {
        public IEnumerable<Artikel> Read(byte[] artikelXml) {
            yield break;
        }
    }
}