WordPress installation in Joomla subdirectory

I’ve a Joomla 1.5.26 installation on the root of my hosting space. I would like to install a wordpress site in a subdirectory.

So I’ve edited the .htaccess file in the root adding these two lines immediately after RewriteEngine On:

RewriteCond %{REQUEST_URI} ^/subdirectory/
RewriteRule ^(.*)$ $1 [L]

I’ve edited also the functions.php file adding these two line after php opening tag (obviously using my domain and subdirectory name):

update_option('siteurl', 'http://www.example.com/subdirectory');
update_option('home', 'http://www.example.com/subdirectory');

I’ve changed the “home” and “siteurl” field value in the table wp_options in the mysql database with “http://www.example.com/subdirectory“.

When I try to access the subdirectory I get an HTTP 500 error.