supported mscorlib types

Hello, I am testing typemock.

I am trying to do something like:
var fileStream = Isolate.Fake.Instance<FileStream>();

But am getting an exception saying:
"TypeMock.TypeMockException: *** Cannot mock types from mscorlib assembly."

However, there is a list (http://site.typemock.com/mscorlib-types) where you said it is supported (as well as you do with MemoryStream, which throws the same exception..)

I've got the latest version (downloaded yesterday).

Am I doing something wrong?

PS. I cannot create a wrapping class, as suggested in the forum, as the code is currently in production.

Thanks!