Custom Assert "IS" Enum

Hi all,

according to

https://www.typemock.com/docs/?book=Ipp&page=custom_assertions_with_predica.htm

i am trying to us the IS<> functionality to check in an assertion, if a function was called with the expected argument. The argument is an enum. I get the following error when trying to run the test:

C++ exception with description "ERROR: Internal Error in predicate - no arg copied" thrown in the test body.

However, if I use for example a std::string type, the assertion works fine. What could be the issue concerning the enum? Thank's a lot in advance.