print to file device

[Sucs]

Dear Michael Sweet,
   I have a trouble with cups.
   I want use cups to print to file device, but failed.
   The follow is my steps.
             1. add a line "FileDevice Yes" to the cupsd.conf file
             2. restart cups service
             3. add a printer via command:
                   lpadmin -p test -E -v file:~/test.prn
                      -m /usr/share/cups/model/Lexmark/Lexmark.ppd
             4. type the button of "Print Test Page",
                    there is a error msg
"Unable to open output file "file:/root/test.prn" - Permission denied."
and the job stopped
   What steps do i lack?
   Thank for your help in advance