Isolator 5.1 - WillThrow(new TimeoutException())

Per the subject, I'm testing some WCF-related code and trying to setup an exceptional case:

Isolate.SwapNextInstance<AutofacServiceHost>().With(autofacHost);
Isolate.WhenCalled(() => autofacHost.Open()).IgnoreCall();
Isolate.WhenCalled(() => autofacHost.Close()).WillThrow(new TimeoutException());

The code under test:

try
{
    serviceHost.Close();
}
catch (CommunicationObjectFaultedException)
{
}
catch (TimeoutException)
{
}

The exception is thrown, but if I step through it in the debugger the type of the exception object is System.MulticastDelegate, not System.TimeoutException. Very bizarre...

If you change the above isolation to simulate a CommunicationObjectFaultedException, everything works as expected.

Any thoughts?

Thanks,
Matt