The requirements were not consistant with the answer for the Test. None != NaN. math library has an nan which should have been the correct answer.

Yeah, I just suspect they didn't want to change the wording on the question and techincally you don't need the destination parameter but then the function would have to either alter str in place or allocate memory in the function and free it outside. It's easier to just do it the way they have, though I like the idea of altering the string in place.