Refile targets fail with certain filename

[Nathan Neff <nathan.neff@gmail.com>]

Hello,

This was a very tricky problem to work around.  I have two directories
("notesmine" and "personal") that I use to refile stuff to.  I refile to my
notesmine directory once every few months, so I was baffled last night
when I started getting "Wrong type argument:
stringp, nil" when I wanted to refile to my notesmine directory.

;; Setup
;; My /Users/nate/Documents/notesmine directory has a file called
"org-mode.org" in it.

;; Begin Code
(defun njn/personal-org-files()
       (interactive)
       (directory-files "/Users/nate/Documents/notesmine" 't "^[^.].*org$")
    )

(defun njn/refile-targets-personal()
    (interactive)
    (setq org-refile-targets
          (quote ((njn/personal-org-files :maxlevel . 5))))
  )

;; I would then run M-x njn/refile-targets-personal, and
;; then press Ctrl-C Ctrl-W to refile a heading in one of my org files.
;; Then, I would see the "Wrong type argument: stringp nil"

I found that the problem *disappears* when I remove or re-name
the "org-mode.org" file from my notesmine directory!

I refile to my personal directory all the time, and I found that if
I move the "org-mode.org" file to my personal directory, (or create a blank
"org-mode.org" file in my personal directory then I would start
getting the same "Wrong type argument stringp, nil" error when refiling
to my personal directory.

So, as a geek, I'm interested in why the "org-mode.org" filename causes
this error.  I've reproduced the problem with the simple code above, so
I hope someone else can say "Yes, I can reproduce the problem" and I will
have done some good :-)

Can anyone reproduce the error and tell me if "org-mode.org" is a keyword or
function or something, or is the problem in the regular expression I'm
using to select
my refile targets?

Thanks,
--Nate