Call to a member function quote() on null

        {
        $user_id = $user->id;
        $new_username = $data['com_ep_companyname'];
        if (!empty($new_username))
            {
            $query = 'UPDATE #__users SET username = ' . $db->quote($new_username) . ' WHERE id = ' . $user_id;
            $db = JFactory::getDbo();
            $db->setQuery($query);
            $db->execute();
            }
        }