Loading a file vs evaluating its buffer

[Thorsten Jolitz]

Hi List, 

when I try to store the installation directory of an app while loading
its init file "A.el" with this code

#+BEGIN_SRC emacs-lisp
(defvar my-installation-directory ""
 "My installation directory.")

(setq my-installation-directory
      (file-name-directory
       (directory-file-name
        (file-name-directory
         (buffer-file-name)))))
#+END_SRC

I get 

,----
| Load error for /home/tj/junk/el/A.el:
| (wrong-type-argument stringp nil)
`----

but doing M-x eval-buffer in buffer "A.el" works as expected:

,----[ C-h v my-installation-directory RET ]
| my-installation-directory is a variable defined in `A.el'.
| Its value is "/home/tj/junk/"
| 
| Documentation:
| My installation directory.
`----

How can I get the same result when loading the file?

-- 
cheers,
Thorsten