nnmail-split bug?

[Norman Walsh <ndw@nwalsh.com>]

In an effort to get more complete threads, I've started to CC
myself on mail. I want these messages to go into the right
folder, regardless of what split criteria got the initial
message into its folder. So I added a header (you can see it
above, I'm sure)

  X-local-group-name: 

and I stick the name of the group that's active when I'm sending mail
onto that header.

Then I added the following rule to the beginning of my
nnmail-split-methods:

	("\\1" 
	 			"^X-local-group-name: \\(.*\\)")

Problem is, when this rule fires, gnus goes "bang".

Q1. Is there a better way to achieve what I'm after?

Q2. Here's the debug backtrace.  Any ideas how to fix it?  Is it user
    error?  This is 5.6.43 under Emacs 19.34.6 on NT.

Signaling: (args-out-of-range 685 730)
  replace-match("\\1" nil nil "\\1")
  nnmail-article-group(nnml-active-number)
  nnmail-check-duplication("<9176-Fri20Nov1998093858-0500-nwalsh@arbortext.com>" nnml-save-mail nnml-active-number)
  nnmail-process-unix-mail-format(nnml-save-mail nnml-active-number)
  nnmail-split-incoming("~/.gnus-crash-box" nnml-save-mail nil "mail.misc" nnml-active-number)
  nnmail-get-new-mail(nnml nnml-save-nov "~/Mail/" "mail.misc")
  nnml-request-scan("mail.misc" "")
  gnus-request-scan("nnml:mail.misc" (nnml ""))
  gnus-activate-group("nnml:mail.misc" scan)
  gnus-get-unread-articles(2)
  gnus-group-get-new-news(2)
  call-interactively(gnus-group-get-new-news)