Compilation error with gnus-topic-line-format

[Olivier Sirven <the.slaa@gmail.com>]

Hello,

I'm willing to customize gnus-topic-line-format using a custom face
declared like this:

--8<---------------cut here---------------start------------->8---
(copy-face 'bold 'my/face/unread-counter)
(set-face-foreground 'my/face/unread-counter "tomato")
(setq gnus-face-1 'my/face/unread-counter)

(setq gnus-topic-line-format "%i[ %(%{%n%}%) -- %1{%A%} ]%v\n")
--8<---------------cut here---------------end--------------->8---

But when I restart gnus to take advantage of it, I get a broken layout
with the following compile log:

,----
Warning: `(princ total-number-of-articles)' is a malformed function
`----

Here is the gnus-update-format output:
,----
(progn
  (insert indentation "[ ")
  (gnus-put-text-property
   (point)
   (progn
     (gnus-add-text-properties
      (point)
      (progn
        (insert name)
        (point))
      '(gnus-face t face bold))
     (point))
   gnus-mouse-face-prop gnus-mouse-face)
  (insert " -- ")
  (gnus-add-text-properties
   (point)
   (progn
     ((princ total-number-of-articles))
     (point))
   '(gnus-face t face my/face/unread-counter))
  (insert " ]" visible "\n"))
`----

What am I doing wrong here?

For the record, my emacs version is GNU Emacs 23.2.1 (i486-pc-linux-gnu,
GTK+ Version 2.20.1) of 2010-09-02 on rutherfordium, modified by Debian
and my gnus version is Gnus v5.13

Thanks

--
Olivier