InvalidOperationException on subject

I'm wirting some integration test for our application. I'm using mspec and got a invalid operation exception on subject.

using FluentAssertions;
using Machine.Fakes;
using Machine.Specifications;
using Mehdime.Entity;
using Castle.Core.Internal;

namespace Bachem.Dispo.BusinessLogic.Tests
{
[Subject(typeof(MitarbeiterManager))]
public class MitarbeiterManagerSpecification : WithSubject<MitarbeiterManager>
{
    protected static DispoDataContext DataContext;
    protected static MitarbeiterManager mitarbeiterManager;
    protected static int mitarbeiterId;

    Establish context = () =>
    {
        With(new DropAndCreateDatabaseBehaviorConfiguration());

        Configure(x => x.For<IDbContextScopeFactory>().Use<DbContextScopeFactory>());
        Configure(x => x.For<IDbContextFactory>().Use<DbContextFactory>());
        Configure(x => x.For<IMitarbeiterRepository>().Use<MitarbeiterRepository>());

        MappingConfig.Initialize();

        mitarbeiterId = 1339;

        var mitarbeiter = new Mitarbeiter
        {
            Id = mitarbeiterId,
            Vorname = "Hans",
            Nachname = "Peter",
            Personalnummer = 1337,
            Anrede = 1,
            Postleitzahl = "33100",
            Strasse = "Hauptstrasse 32",
            Ort = "Paderborn",
            Telefonnummer = null,
            Email = null,
            Mobilnummer = null
        };

        using (var context = DataContextFactory.CreateDataContextStatic())
        {
            context.Mitarbeiter.Add(mitarbeiter);
            context.SaveChanges();
        }
    };
}

public class Can_get_Mitarbeiter : MitarbeiterManagerSpecification
{
    protected static MitarbeiterDto[] result;

    Because of = () =>
    {
        result = Subject.GetMitarbeiter();
    };

    It it_should_be_hans_peter = () =>
    {
        result.Find(r => r.Id == mitarbeiterId).Id.Should().Be(mitarbeiterId);
    };
}
}

The exception only tells me that a invaild operation is used on subject. Subject comes from machine fakes.
The test should add a new "Mitarbeiter" to the database and load this "Mitarbeiter" and check for equality.
The method used to load "Mitarbeiter" returns an array of type "MitarbeiterDto"