MockObject on a mixed wrapper doesn't work

Hi all,
I'm getting a System.NullReferenceException while trying to mock a mixed wrapper class.
The wrapper class has a private constructor. The constructor has only one parameter, a pointer to the unmanaged wrapped object. Something like this:

_gc public class Wrapper
{
  private:
    Wrapper(MyUnmanagedClass__nogc *ptr);
    [...]
}

The exception is raised when executing the following statement in the setup of a fixture:

  MockObject mockWrapper = MockManager.MockObject(typeof (Wrapper));

Is this a bug or a not supported features? Is there any fix/workaround for this problem?

Thank you

P.S.: Mocking the type doesn't raise any exception (using MockManager.Mock() method)