WordPress MU

I'm trying to load WordPress from my application using:
define('WP_USE_THEMES', false);
require( dirname(__FILE__) . '/wp-blog-header.php' );

but I'm getting an error with $wpdb being undefined when it reaches is_blog_installed in functions.php

If I var_dump $wpdb in wp_settings.php just before the call, $wpdb is ok.

Inside functions.php, even though the is_blog_installed has global $wpdb at the beginning, the variable is null - causing execution to exit.

Any idea what I might be doing wrong?

http://wordpress.org/support/

This is the MU forums.

I am using MU.

Then you should have specified that.

does $wpdb->options have a value a that point for you?