Error when assigning to a property aliased to a property of type Component

import QtQuick 1.1

Rectangle {
    id: root
    width: 400
    height: 400
    property alias sourceComponent: loader.sourceComponent

    Loader {
        id: loader
    }

    MouseArea {
        anchors.fill: parent
        onClicked: {
            loader.sourceComponent = undefined // OK
            root.sourceComponent = undefined // error
        }
    }
}