Error when restarting gnus

[Vin Shelton <acs@xemacs.org>]

I recently added nnfolder to my list of backends.  If I let gnus sit
idle for an hour or so, then quit gnus and restart it, here is the
backtrace I get:


Signaling: (wrong-type-argument stringp nil)
  insert-file-contents-internal(nil nil nil nil nil nil nil)
  insert-file-contents(nil)
  byte-code("..." [file coding-system-for-read auto-mode-alist file-name-coding-system nnmail-pathname-coding-system nnmail-file-coding-system mm-auto-mode-alist insert-file-contents t] 3)
  nnmail-find-file(nil)
  nnfolder-request-list(nil)
  gnus-request-list((nnfolder))
  gnus-read-active-file-1((nnfolder) nil)
  gnus-read-active-file(nil nil)
  gnus-setup-news(nil nil nil)
  byte-code("..." [gnus-slave gnus-current-startup-file gnus-startup-file did-connect level dont-connect gnus-group-quit gnus-run-hooks gnus-startup-hook gnus-make-newsrc-file gnus-dribble-read-file bbb-login add-hook gnus-summary-mode-hook gnus-grouplens-mode gnus-setup-news nil gnus-setup-news-hook gnus-start-draft-setup gnus-group-list-groups gnus-group-first-unread-group gnus-configure-windows group gnus-group-set-mode-line gnus-started-hook gnus-use-dribble-file gnus-use-grouplens] 4)
  gnus-1(nil nil nil)
  gnus(nil)
  call-interactively(gnus)
  command-execute(gnus t)
  execute-extended-command(nil)
  call-interactively(execute-extended-command)


I'm running a gnus built from CVS on July 9, under XEmacs 21.4.
Here's the part of my .gnus.el file that deals with nnfolder:

(setq nnfolder-directory "~/Mail/archive")
(setq gnus-parameters
      '(("nnfolder:xemacs-patches\\..*"
	 ; Setting gnus-visible-headers has no effect.
	 ; Ideally, we would just add Message-id to gnus-visible-headers,
	 ; so I can use 'msgid' to save the message.
	 ; I'll make all headers visible instead (yuck!).
;	 (gnus-visible-headers "^From:\\|^Newsgroups:\\|^Subject:\\|^Date:\\|^Summary:\\|^Keywords:\\|^To:\\|^[BGF]?Cc:\\|^Posted-To:\\|^Resent-From:\\|^X-Sent:\\|^Message-[Ii][Dd]:")
	 (gnus-show-all-headers t)
	 (gnus-large-newsgroup 2000)
	 (display . all))))
(setq gnus-move-split-methods
      '(((equal gnus-newsgroup-name "gnu.emacs.sources") "nnml:emacs")
	((equal gnus-newsgroup-name "rec.humor.funny") "nnml:humor"))
      nnmail-spool-file acs::mailspool
      nnmail-keep-last-article nil	; t if using exmh
;      nnmail-delete-incoming nil	; save a copy of incoming mail
      gnus-secondary-select-methods
      '((nnml "")
	(nnfolder))
      nnmh-get-new-mail nil
      nnmh-be-safe t
      nnml-get-new-mail t
      gnus-treat-display-smileys nil	; don't smilify by default
      gnus-large-newsgroup 1000)

What am I doing wrong?

Thanks in advance,
   Vin