error:...out of order

Dear all,

I have put the following in my .gnus.el in order to save sent mail:

(setq gnus-message-archive-method
      '(nnmbox   "archive"
          (nnmbox-mbox-file "~/mail/sent-mail")
          (nnmbox-get-new-mail nil)))
(setq gnus-message-archive-group "sent-mail")

The first time I send a mail from a gnus session, I get the following
error.

    Article mail.misc:1190 out of order

The mail is sent but not saved in the file sent-mail.
Strange enough, if I send again mails, I get no error
and they are saved correctly. What does the error mean and what can I
do to get rid of it ?
Thanks