Active Record error with order_by random

[eluser]fatdog[/eluser]
I'm getting this error.

Unknown column ' RAND() LIMIT 1' in 'order clause'
SELECT * FROM (`camp`) WHERE `secc` = 7 ORDER BY ` RAND() LIMIT 1

Code:

$this->db->order_by("id","random");
$this->db->where('secc',7);
$this->db->limit(1);
$query = $this->db->get('camp');

Am I doing something wrong?

I'm using version 1.7.0