Setting orderby to a custom field using pre_get_posts

Not sure exactly why, but by adding another filter using the meta_query it all started working. I now only get posts in my custom post type archive that do not have the project_update field set and are ordered by project_status. This is what I was after.

$meta_query = array(
    array(
       'key'=>'project_update',
       'value'=>'1',
       'compare'=>'!=',
    )
);

$query->set('meta_query', $meta_query);