Unexpected Animation "running" property behavior

import QtQuick 1.1
Item {
    width: 950
    height: 500

    Rectangle {
        id: blueRect
        color: "blue"
        width: 500; height: 300

        Rectangle {
            id: redRect
            color: "red"
            width: 50
            height: 50
            anchors.verticalCenter: parent.verticalCenter
        }

        SequentialAnimation {
            id: anim
            running: true                  // works
//          running: blueRect.width > 0    // doesn't work!
            onRunningChanged: print("running: " + running)
            loops: Animation.Infinite
            NumberAnimation {
                target: redRect; property: "x"
                from: 0; to: blueRect.width-redRect.width
                duration: 1000
            }
            NumberAnimation {
                target: redRect; property: "x"
                from: blueRect.width-redRect.width; to: 0
                duration: 1000
            }
        }
    }
}