Cannot set the property type to a custom Q_GADGET object

Window {
    id: root

    property ValueType vt
    property var vt2

    width: 500
    height: 500
    visible: true
    Component.onCompleted: {
        console.log(root.vt) // Prints null
    }

    Playground {
        id: pl
    }

    Button {
        text: "10"
        onClicked: {
            // Doesn't work
            root.vt = pl.value()
            // Works
            root.vt2 = pl.value()
        }
    }
}