https://bugs.winehq.org/show_bug.cgi?id=53845
--- Comment #4 from Nikolay Sivov bunglehead@gmail.com --- (In reply to Javier from comment #3)
I'm not particularly an expert in this function, but I'm not even sure if the implementation should check for that value in this case, considering that it is being passed CAL_RETURN_NUMBER, which means it is not going to return a string anyway.
No, that's a modifier for return value. CALID is a calendar type to use, GREGORIAN_US appears to be a neutral one, if I'm reading make_unicode script correctly.
I'm not 100% sure though.