Rhino-Mocking ExpectationViolationException

I started to use Rhino-Mocks and Unit-Tests a few days ago so I'm new in this.

I created a disposable class like this:

public class SomeClass : IDisposable
{
    private bool _disposed;

    private SomeOtherClass ObjB { get; set; }

    public SomeClass(SomeOtherClass b)
    {
        ObjB = b;
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed)
            return;

        if (disposing)
        {
            // set all properties to null

            ObjB = null;
            // ...
        }

        _disposed = true;
    }
}

and the unit test which should check if the dispose was done correctly:

[TestFixture]
public class SomeClassTests
{
    [Test]
    public void ShouldDisposeCorrectly()
    {
        var classB = MockRepository.GenerateStrictMock<SomeOtherClass>()
        SomeClass smth;

        using (smth = MockRepository.GenerateStrictMock<SomeClass>(classB))
        { }

        smth.AssertWasCalled(p => p.Dispose());
    }
}

Now when I start the test it throws the following error: Rhino.Mocks.Exceptions.ExpectationViolationException : SomeClass.Dispose(True); Expected #0, Actual #1.

It makes sense, because Dispose() calls Dispose(boolean) but it's protected so I can't access it in the AssertWasCalled method...

Can you help me to find the missing step? :-)