SDL Audio ConvertAudio fails, SDL_GetError empty

I’m trying to open a wav file and convert it. I’ve set everything up just like in the docs, but it’s failing, but without giving any error messages. Below is my code:
Thanks for any help!

It didn’t fail, please see:

Function returns 0 on success and negative on failure.

Also, SDL_BuildAudioCvt returns 0 if conversion is not needed.