Patrick,
This seems to be a bug in out parameter handling on fakes. I am logging it and we'll fix it in a future version. As a work around you can instantiate FakedClass either using a constructor, or using Isolate.Fake.Instance<FakedClass>(Members.CallOriginal) - these options will cause the instance to behave like a normal FakedClass instance, except for the methods put through WhenCalled().
Thanks,
Doron
Typemock Support