lpadmin can not use specified PPD file

[anonymous]

I used lpadmin -m /home/guest/file.ppd to install a printer and always get
lpadmin: add-printer (set model) failed: server-error-internal-error.

I checked /var/log/cups/error_log, it says
add_printer: Unable to copy PPD file from 
/usr/share/cups/model//home/gusst/file.ppd to /etc/cups/ppd/file.ppd - No 
such file of directory!

Why can't lpadmin always require the directory prefix of 
/usr/share/cups/model?