Shapes do not work with OpacityMask if invisible

import QtQuick 2.4
import QtQuick.Window 2.4
import QtGraphicalEffects 1.0
import QtQuick.Shapes 1.0

Window {
    visible: true
    width: 640
    height: 480
    id: root
    color: "green"

    Item {
        id: i
        width: parent.width * 0.2
        height: width
        anchors.right: parent.right
        anchors.top: parent.top

        Rectangle {
            id: a
            visible: false
            anchors.fill: parent
            color: 'blue'
        }

        Rectangle {
            id: b
            anchors.fill: parent
            color: "transparent"
            visible: true

            Shape {
                id: c
                vendorExtensionsEnabled: false

                ShapePath {
                    id: c_sp
                    strokeWidth: -1
                    fillColor: Qt.rgba(1,0,0,0.5)

                    property point center: Qt.point(b.x + b.width * 0.5 , b.y + b.height * 0.5)
                    property real quarter: b.width * 0.25


                    property point top: Qt.point(center.x, center.y - quarter )
                    property point bottomLeft: Qt.point(center.x - quarter, center.y + quarter )
                    property point bottomRight: Qt.point(center.x + quarter, center.y + quarter )

                    startX: center.x;
                    startY: center.y + quarter

                    PathLine { x: c_sp.bottomLeft.x; y: c_sp.bottomLeft.y }
                    PathLine { x: c_sp.top.x; y: c_sp.top.y }
                    PathLine { x: c_sp.bottomRight.x; y: c_sp.bottomRight.y }
                    PathLine { x: c_sp.center.x; y: c_sp.center.y + c_sp.quarter }
                }
            }
        }


        OpacityMask {
            visible: true
            anchors.fill: a
            source: a
            maskSource: b
            invert: false
        }
    }
}