problem with stds::transform

Hello all, I have the following line in my code:

std::transform(CheckString.begin(), CheckString.end(), CheckString.begin(), char_toupper); // make path all caps

And Borland gives me this error:
Error E2285 checkowner.cpp 52: Could not find a match for 'std::transform<InputIterator,OutputIterator,Unary Operation>(char *,char *,char *,undefined)' in function checkRights(string)

It seems to be somehow "missing" one of the parameters I pass it!

i can't see too much from this but it seams like char_toupper was declared prior but was not initialized ....(defined) or you haven't assigned any value to it...

hmm.....

i guess more code is needed.....mistake is probably in declaration or definition somewhere prior.....