IS NOT NULL using database helper

[eluser]Huji[/eluser]
Hi. When I wanted to get the number of rows in a database with their "surname" field still empty, I simply ran:

$query = $this->db->get_where('users', array('surname' => Null));
$data['reg_count'] = $query->num_rows;

Now I want to do the opposite: Get the number of rows that have their surname field filled with text. In terms of SQL syntax, the above example procudes "SELECT * FROM (`users`) WHERE `surname` IS NULL". What I want now is something like "SELECT * FROM (`users`) WHERE `surname` IS NOT NULL". However, I want to use database helper for that rather than hardcoding it myself. How can I do that?