Cannot query database in custom_helper?

[eluser]java[/eluser]
I have a function placed in custom_helper file :

Code:

function getCityName($id)
{
    $query = $this->db->getwhere('thanhpho', array('ThanhPhoID' => $id));
    $row = $query->row();
    return $row->Ten;
}

i want it automatically called in my view. But the view generated error:

Fatal error: Call to a member function on a non-object in d:\www\hoahau\system\application\helpers\custom_helper.php on line 181

and line 181 is my query command in function above.
What's the wrong in my code? Please tell.
Thanks for any response