SDG and inverse function theorem

Still not sure about status of inverse function theorem in synthetic differential geometry, but at least have found mention of it in one of Anders Kock's books. However, the reference is to an unpublished thesis, and I don't understand enough of the context yet to figure out what the difficulty is with inverse functions. If all functions are microlinear, there shouldn't be a problem, should there? Or is micro to local too big a jump?