Image fails to load if only sourceSize.height or width is specified

By using the example at the bottom of this description, an error is reported on the console:
QML QQuickImage: Failed to get image from provider: image://scaling/0.444444//home/nerochiaro/projects/phone/camera/camera-app-desktop-camera/assets/zoom_plus@18.png
(path is obviously specific to that test run, but the image mentioned in the error actually exists on disk)

Either removing sourceSize.height or adding sourceSize.width makes the error disappear and the image loads correctly.

So it seems that the two sourceSize properties need to be either both set or both unset, but it can't be just one.

import QtQuick 2.0
import Ubuntu.Components 0.1

Item {
 width: 600
 height: 600

    Image {
        anchors.centerIn: parent
        source: "assets/zoom_plus.png"
        sourceSize.height: units.gu(2) // if I remove this it all works just fine
    }
}