I'm having issues using imap as my "sent" archive.  My gnus-server-alist
on gnus startup is like this:-

,----
| gnus-server-alist is a variable defined in `gnus.el'.
| Its value is 
| (("archive" nnfolder "archive"
|   (nnfolder-directory "~/.emacs.d/Maildir/archive")
|   (nnfolder-active-file "~/.emacs.d/Maildir/archive/active")
|   (nnfolder-get-new-mail nil)
|   (nnfolder-inhibit-expiry t)))
`----

despite my .newsrc.eld containing:-

,----
| (setq gnus-server-alist '(("archive" nnimap "mymail" (nnimap-address "offlineimap") (nnimap-stream nil))))
`----

If I then set gnus-archive-message-group to "INBOX.sent" for example I
get a debug stack when I start gnus of the form:-


,----
| Debugger entered--Lisp error: (wrong-type-argument stringp nil)
|   file-name-as-directory(nil)
|   nnheader-concat(nil "active")
|   (defvar nnfolder-active-file (nnheader-concat nnfolder-directory "active") ("/usr/share/emacs/23.1/lisp/gnus/nnfolder.elc" . 1885))
|   require(nnfolder)
|   gnus-get-function((nnfolder "archive" (nnfolder-directory "~/.emacs.d/Maildir/archive") (nnfolder-active-file "~/.emacs.d/Maildir/archive/active") (nnfolder-get-new-mail nil) (nnfolder-inhibit-expiry t)) server-opened)
|   gnus-server-opened((nnfolder "archive" (nnfolder-directory "~/.emacs.d/Maildir/archive") (nnfolder-active-file "~/.emacs.d/Maildir/archive/active") (nnfolder-get-new-mail nil) (nnfolder-inhibit-expiry t)))
|   gnus-check-server((nnfolder "archive" (nnfolder-directory "~/.emacs.d/Maildir/archive") (nnfolder-active-file "~/.emacs.d/Maildir/archive/active") (nnfolder-get-new-mail nil) (nnfolder-inhibit-expiry t)))
|   gnus-read-active-file-1((nnfolder "archive" (nnfolder-directory "~/.emacs.d/Maildir/archive") (nnfolder-active-file "~/.emacs.d/Maildir/archive/active") (nnfolder-get-new-mail nil) (nnfolder-inhibit-expiry t)) nil)
|   gnus-read-active-file(nil nil)
|   gnus-setup-news(nil nil nil)
|   byte-code("„
|   gnus-1(nil nil nil)
|   gnus(nil)
|   call-interactively(gnus nil nil)
`----

Any suggestions?  Why is it trying to open nnfolder for "archive"?