Setting an anchor to undefined does nothing

import QtQuick 2.0

Rectangle {
    width: 400
    height: 400
    visible: true

    Rectangle {
        id: rect
        x: 100
        y: 100
        width: 32
        height: 32
        color: "salmon"

        Text {
            id: textItem
            text: "32"
            anchors.horizontalCenter: parent.horizontalCenter
            anchors.bottom: checkBox.checked ? parent.bottom : undefined
        }
    }

    Text {
        text: "anchors.bottom = " + (checkBox.checked ? "parent.bottom" : "undefined")
        id: checkBox
        anchors.horizontalCenter: parent.horizontalCenter

        property bool checked: false

        MouseArea {
            anchors.fill: parent
            onClicked: parent.checked = !parent.checked
        }

        Rectangle {
            anchors.fill: parent
            color: "transparent"
            border.color: "red"
        }
    }
}