Hi,
Unfortunately, you have stumbled on a corner case object Isolator has an issue with :(. When faking an abstract object, Isolator tries to extend it and create an instance of the new type. In the case of Member property, the SPPrincipal class is both abstract, and does not have an available contructor, thus Isolator fails to intantiate and fake it. This is a known issue, and I'll try to find a workaround for your case.
Doron
Typemock Support