Fetching scaler value

How can I fetch a scaler value from DB. Using following return Fatal error: Undefined class constant 'FETCH_COLUMN' in... error.

$this->di['db']->fetchOne('select * from user where username=:username and password=:password', Phalcon\Db::FETCH_COLUMN,
                ['username' => $request['username'], 'password' => $request['password']]);