Unable to access external DB with $wpdb

I’ve found older threads about this, but I can’t seem to get the following code to work at all:

$mydb = new wpdb(‘username’,’password’,’database’,’localhost’);
$querystr =”
SELECT $mydb->posts.* FROM $mydb->posts
WHERE $mydb->posts.post_type = ‘post’
ORDER BY $mydb->posts.post_date DESC
LIMIT 5″;
$pageposts = $mydb->get_results($querystr, OBJECT);

When I parse $pageposts, I get nuthin’.

The same code works fine with the default $mydb:

global $wpdb;
$querystr =”
SELECT $wpdb->posts.* FROM $wpdb->posts
WHERE $wpdb->posts.post_type = ‘post’
ORDER BY $wpdb->posts.post_date DESC
LIMIT 5″;

$pageposts = $wpdb->get_results($querystr, OBJECT);

The above, with the default $wpdb, works fine. Can anybody help with this?