[Orgmode] Invalid capture target specification

[Thorsten]

I try to set up Org capture with a default target file. 

from my .emacs:
; set file and global key for org-capture
(setq org-directory "/home/tj1/Dropbox/.emacs.d/org")
(setq org-default-notes-file (concat org-directory "/notes.org"))
(define-key global-map "\C-cc" 'org-capture) 

C-h v org-default-notes-file gives:
org-default-notes-file is a variable defined in `org.el'.
Its value is 
"/home/tj1/Dropbox/.emacs.d/org/notes.org"

but C-c c gives:
org-capture-set-target-location: Invalid capture target specification

To me it looks as if I just followed the manual and everything should be fine. 
I 
do not need a capture template, don't I?
I would appreciate any advice. 

Thorsten

PS 
GNU Emacs 23.1.1 (i486-pc-linux-gnu, GTK+ Version 2.20.0) of 2010-03-29 on 
rothera, modified by Debian
Org-mode version 7.01h