Why different behaviour of concat on OSX and on Lubuntu

[Guido Van Hoecke <guivho@gmail.com>]

Hi,

My main machine is an iMAC (querty keyboard) running GNU Emacs 24.3.1
(x86_64-apple-darwin, NS apple-appkit-1187.37)

I have to use temporarily an old notbook (belgian azerty keyboard)
running Lubuntu GNU Emacs 24.2.1 (i686-pc-linux-gnu, GTK+ Version
3.6.4) of 2013-04-09 on komainu, modified by Debian.

I use a seldom used key to create some mappings.

Part of the code looks like this:
;; check whether running on OSX
(setq isOSX (equal (expand-file-name "~/") "/Users/guivho/"))

(if isOSX
    (setq leader "§")
  (setq leader "²"))

(global-set-key (kbd (concat "M-" leader)) 'overwrite-mode)

The last statement maps overwrite-mode to the M-§ sequence on my iMac.

On the Lubuntu machine, it does not map M-² to overwrite-mode, It barks:

(wrong-type-argument integer-or-marker-p (concat "M-" leader))
  read-kbd-macro((concat "M-" leader)

On the other hand, the following commands do work on both machines:

(define-prefix-command 'guivho-map nil "guivho-")

(define-key guivho-map    leader 'guivho-save-and-kill-buffer)
(define-key guivho-map    "-" 'hl-line-mode)
(define-key guivho-map    "=" 'vc-diff)
(define-key guivho-map    "0" 'delete-window)
...
(define-key guivho-map "z" (lambda() (interactive)
(switch-to-buffer-other-window "*scratch*" t)))
(global-set-key leader guivho-map)

I am puzzled why the concat does not work on lubuntu, and would like
to know how to code it to achieve the desired effect.

Thanks in advance,

Guivho.