Extend-property in State-element has no effect

import Qt 4.7


Rectangle {
    width: 200
    height: 200

    Rectangle {
        id:box
        width:100
        height:100
        color:"red"
        states:[
            State {
                name: "a"
                changes:PropertyChanges {
                    target: box
                    x:50
                }
            },
            State {
                name: "b"
                extend:"a"
                changes:PropertyChanges {
                    target: box
                    y:50
                }
            }
        ]

    }

    MouseArea {id:mouse;anchors.fill: parent; onClicked:box.state="b"}

}