Dialog of QtQuick.Dialogs not valid

can't use Dialog of QtQuick.Dialogs in qml.

Archlinux/KDE

command: qml6 a.qml 

result:
QQmlApplicationEngine failed to load component
file:///tmp/a.qml:9:2: Dialog is not a type
qml: Did not load any objects, exiting.

code:

import QtQuick.Controls
import QtQuick.Dialogs as Dlg

ApplicationWindow {
	visible: true
	width: 800
	height: 480
	Button {
		text: "open"
		onClicked: dlg.open()
	}
	Dlg.Dialog {
		id: dlg
	}
}