Setting a negative width and height on a ShaderEffectSource or DropShadow causes assertion failure

import QtQuick 2.0
import QtGraphicalEffects 1.0

Rectangle {
    width: 480;
    height: 300;

    Canvas {
        id: canvas
        anchors.centerIn: parent
//        width: -5
//        height: -5
        width: parent.width - arcLineWidth * 2
        height: parent.height - arcLineWidth * 2

        property real radius: Math.min(width, height) / 2 - arcLineWidth
        property real arcLineWidth: 10

        onPaint: {
            var ctx = getContext("2d");
            ctx.reset();

            ctx.beginPath();
            ctx.arc(width / 2, height / 2, radius, 0, Math.PI, false);
            ctx.closePath();

            var gradient = ctx.createLinearGradient(0, 0, 0, radius * 2);
            gradient.addColorStop(0, "#ff0000");
            gradient.addColorStop(1, "#ffaaaa");
            ctx.lineWidth = arcLineWidth;
            ctx.strokeStyle = gradient;
            ctx.stroke();
        }
    }

    ShaderEffectSource {
        id: canvasSource
        sourceItem: canvas
        anchors.fill: sourceItem
//        width: -5
//        height: -5
        hideSource: true
    }

    DropShadow {
        source: canvas
        anchors.fill: source
//        width: -5
//        height: -5
        radius: 8
        samples: 16
    }
}

void QSGDefaultImageNode::updateGeometry()
{
    Q_ASSERT(!m_targetRect.isEmpty());
    const QSGTexture *t = m_material.texture();
    if (!t) {

0	__GI_raise	raise.c	64	0x7ffff4edd425	
1	__GI_abort	abort.c	91	0x7ffff4ee0b8b	
2	qt_message_fatal	qlogging.cpp	944	0x7ffff58fd11b	
3	QMessageLogger::fatal	qlogging.cpp	360	0x7ffff58fad8e	
4	qt_assert	qglobal.cpp	1992	0x7ffff58f665c	
5	QSGDefaultImageNode::updateGeometry	qsgdefaultimagenode.cpp	392	0x7ffff78c9197	
6	QSGDefaultImageNode::update	qsgdefaultimagenode.cpp	347	0x7ffff78c8f3c	
7	QQuickShaderEffectSource::updatePaintNode	qquickshadereffectsource.cpp	1025	0x7ffff7a150e2	
8	QQuickWindowPrivate::updateDirtyNode	qquickwindow.cpp	2432	0x7ffff790d00f	
9	QQuickWindowPrivate::updateDirtyNodes	qquickwindow.cpp	2253	0x7ffff790bf73	
10	QQuickWindowPrivate::syncSceneGraph	qquickwindow.cpp	300	0x7ffff7904345	
11	QSGRenderThread::sync	qsgthreadedrenderloop.cpp	530	0x7ffff78d9da3	
12	QSGRenderThread::syncAndRender	qsgthreadedrenderloop.cpp	569	0x7ffff78d9f2b	
13	QSGRenderThread::run	qsgthreadedrenderloop.cpp	664	0x7ffff78da460	
14	QThreadPrivate::start	qthread_unix.cpp	339	0x7ffff590f4d1	
15	start_thread	pthread_create.c	308	0x7ffff3c8fe9a	
16	clone	clone.S	112	0x7ffff4f9acbd	
17	??