I've also run into this, and is terribly annoying during development. Anytime a character is ran through a certain set of functions (IsSpace, and IsNumber) are a few that I can tell, this assertial will happen.
It only happens when compiled in debug, and only if the character has an ascii value of 128 (or 256, can't remember) or above.
I'm not sure of a solution, but I'm pretty sure that should help clarify your problem.