Can't update primary key

i follow your suggest and this my model:

PHP Code:

// update data
    function update($id, $data)
    {    
        $this->db->where('nis', $id);
        $this->db->update($this->table, $data);
    } 


or i try this too:

PHP Code:

// update data
    function update($id, $data)
    {    
        $this->db->where($this->id, $id);
        $this->db->update($this->table, $data);
    } 


it's still not working.