Context2D functions fail if passed extra arguments

import QtQuick 2.0

Rectangle {
    width: 300
    height: 300

    Canvas {
        anchors.fill: parent
        anchors.margins: 10

        onPaint: {
            var ctx = getContext("2d");
            ctx.beginPath();
            ctx.fillStyle = "red";
            ctx.fillRect(0, 0, width, height/*, 0*/);
        }
    }
}