Another Mifare Ultralight C authentication question

Hello,

I'm trying to authenticate ultralight c with NXP's MFRC500.  I've try to following the authenticate method in the libfreefare.  Everytime I send the AFh command together 16 bytes ek(RndA + RndB) with transceive method, I got only one zero byte answer from the ultralight c.   I can't find any information or example from the internet and I've never receive any answer from NXP no matter how many time I send a support request.  Please help.

Thanks!