Switch_to_blog() outside wordpress

how can I load switch_to_blog() function outside wordpress?

$blog_id = '7';
require_once( $_SERVER['DOCUMENT_ROOT'] . '/wp-load.php' );
switch_to_blog($blog_id);
echo get_option('blogname');
restore_current_blog();
echo 'You switched back.';

I get error:

Fatal error: Call to a member function set_prefix() on a non-object in /home/public_html/wp-includes/ms-settings.php on line 126

Any ideas ?

UPDATE It can be loaded but without any function just this code ? What global variables is missing ?