Using wp-blog-header.php outside wordpress

Hi, i’m trying to include some content in a some site outside my current install of WP 2.8.4, but when I do the include

define(‘WP_USE_THEMES’, true);
require($_SERVER[‘DOCUMENT_ROOT’] . ‘/blog/wp-load.php’);

Then I get this error

Fatal error: Call to undefined method stdClass::set_prefix() in /home/../public_html/blog/wp-settings.php on line 268

Can anyone tell how can I solved this, how can I call the prefix to the DB to make this code work?

Thanks for your concern.