bug#4005: 23.1.50; Wrong type argument: characterp ...

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

1. In shell create a new dir and
   - touch k.tex
   - touch .dir-locals.el with the following:

     ((latex-mode . ((fill-column . 78))))

2. Emacs -q k.tex

3. C-h v fill-column RET

Backtrace:
Debugger entered--Lisp error: (wrong-type-argument characterp "/Users/ford/kkt/")
  describe-variable(fill-column)
  call-interactively(describe-variable nil nil)

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
If you would like to further debug the crash, please read the file
/Applications/Emacs.app/Contents/Resources/etc/DEBUG for instructions.


In GNU Emacs 23.1.50.1 (i386-apple-darwin9.7.0, NS apple-appkit-949.46)
 of 2009-07-26 on neutron.local
Windowing system distributor `Apple', version 10.3.949
configured using `configure  '--with-ns''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: nil
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Major mode: LaTeX/MP

Minor modes in effect:
  diff-auto-refine-mode: t
  reftex-mode: t
  LaTeX-math-mode: t
  TeX-PDF-mode: t
  shell-dirtrack-mode: t
  dired-omit-mode: t
  recentf-mode: t
  icomplete-mode: t
  savehist-mode: t
  display-time-mode: t
  xterm-mouse-mode: t
  delete-selection-mode: t
  global-auto-revert-mode: t
  minibuffer-depth-indicate-mode: t
  show-paren-mode: t
  rcirc-track-minor-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t