wrong type argument: stringp, nil

[invokey]

hi, all ,
        I have got some trouble with my emacs to send mail. whenever I C-c
C-c to send a mail under Gnus,
        I always got an error message:
                wrong type argument: stringp, nil

        Yes, I know that might caused by a fake smtp setting, however, I can't
find the bug out.

here are my related lines of .emacs:
(setq gnus-select-method '(nntp "freenews.netfront.net"))
(setq gnus-secondary-select-methods '((nnfolder "~/Mail")))

(setq nnmail-spool-file '(
   (file :path "/var/spool/mail/zzh")
   (directory :path "/home/zzh/Mail" :suffix ".procmail")
   (pop :server "pop.163.com" :user "biozhang2001" :password "mypswd")

))


(setq smtpmail-smtp-server "smtp.163.com")
;; If you use the default mail user agent.
(setq send-mail-function 'smtpmail-send-it)
;; If you use Message or Gnus.
(setq message-send-mail-function 'smtpmail-send-it)

here are my related lines of .gnus:

;; SMTP server
(setq gnus-secondary-select-methods '((nnfolder "/home/zzh/Mail")))
(setq user-full-name "biozhang") ;;
(setq user-mail-address "biozhang2001@163.com") ;;
(setq smtpmail-auth-credentials
'(("smtp.163.com" ;; SMTP server
:port 25
:user "biozhang2001" ;;
:password "mypswd"))) ;;
(setq smtpmail-default-smtp-server "smtp.163.com")
(setq smtpmail-smtp-server "smtp.163.com")
(setq message-send-mail-function 'smtpmail-send-it)

Best Regards

zhang