Problem with update data (reduce value)

[eluser]Carmichael[/eluser]

Code:

function reduce_credits($id, $credits)
   {
        $this->db->where('user_id', $id);
        $this->db->update($this->user_table, 'user_credits = user_credits - '.$credits.'');
   }

When I run that function it updates the user_credits column to 0 insted of reduce the current value. Why?