I must be missing something obvious, or Boost.Mpl doesn't work on MSVC 7.1.

I'm working on my container class C<METRIC, PAYLOAD>. Using MPL I've defined a class static bool IS_OUTER which, if true, means that PAYLOADhas a typedef for the name "key_type". If IS_OUTER is false, then PAYLOADmay not have such a typedef (and in my test case, it doesn't). What I want to do, in C, is have a typedef that is PAYLOAD::key_type if IS_OUTER is true and just plain PAYLOAD if IS_OUTER is false. I cannot make it work. I've checked IS_OUTER and it has the correct value. But no matter what combination of if_c, eval_if_c, apply, etc. I use it won't compile because "'key_type' : is not a member of". Here's my basic example without the elaboration:

         typedef typename boost::mpl::if_c<IS_OUTER,
                  typename PAYLOAD::key_type,
                 PAYLOAD
        >::type testing;

I've tried using eval_if_c with identity<> in the appropriate places. I wrote a metafunction to get key_type and used apply<> on that. In every case, however, the compiler evaluates PAYLOAD::key_type and fails. Is this kind of thing possible using MPL? Is it possible using MPL and MSVC 7.1? If so, can anyone give me a clue? Thanks.