$this->db->update() not working for me

[eluser]Robert May[/eluser]
No idea why, but this query doesn't work (nor does any alteration on it using set() or where()). If you replace update() with insert() it works more as you would expect.

$this->db->update(self::MODEL_TABLE, array('retrieved' => 1), array('id' => $entry->id));

Any ideas as to what's wrong?