nnmail-expiry destroys first archived mail

[DaveS]

emacs 21.3.1 on WinXP
No Gnus v0.2

I've setup a mailing list group to expire article to an archive group:
(defun my-summary-mode-hook-fancy-expiry ()
  (cond
   ;; mythtvusers mailing list
   ((string-match "list\\.mythtvusers$" gnus-newsgroup-name)
    (setq nnmail-expiry-wait 30
          nnmail-expiry-target 'nnmail-fancy-expiry-target
          nnmail-fancy-expiry-targets
          '(("from" ".*" "nnfolder+archive:list.mythtvusers-%Y-%m"))))
   ;; everything else, use the gnus defaults
   (t
    (setq nnmail-expiry-wait 7
          nnmail-expiry-target 'delete
          nnmail-fancy-expiry-targets nil))))
(add-hook 'gnus-summary-mode-hook 'my-summary-mode-hook-fancy-expiry)

This correctly creates a new archive group for each month of mail as it
expires.
But the first message in each month is always corrupted.  Instead of the
message I get:

>From nobody Sun Feb 20 09:26:42 2005
Message-ID: <upsyv2ld9.fsf@totally-fudged-out-message-id>
Lines: 0
Xref: DARTH list.mythtvusers-2004-05:1
X-Gnus-Article-Number: 1   Sun Feb 20 09:26:42 2005


The rest of the messages for that month get archived correctly.  Anyone know
why this is happening?

-- 
"I've seen it.  It's rubbish."
-- Marvin the Paranoid Android