Impossible read and write. I only get the UID

Hi
I'm trying to read and write a mifare classic card with default keys and alwyis get a value error (-2) on each authenticate.
I only get the UID. The ouput when I type ' python mifareauth.py' is
  Reading card 4ec6f455
  0
  1 
  2
  ...

Others apps as 'nfc-mfclassic w a u outputp.mfd' write and read fine.

What is wrong in my pynfc installation?

Thanks in advance
