$this->db->affected_rows() returns -1 after query

[eluser]bleu[/eluser]
I have a query like this

Code:

$update= $this->db->query("update aas set aa = 'aa'  where no=" . $this->db->escape($No) . "");

When I run

Code:

$this->db->affected_rows()  or $this->db->affected_rows($update)

it returns -1

When updating any row which exists I get -1
Even when I have no row to update it still shows -1.

What is the issue? I am using codeigniter 2.1.0 with mysqli drivers