Drill picking not selecting billboards

1. A concise explanation of the problem you're experiencing.

I cannot pick or drill pick billboards that are displaying on my globe.

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

I try to pick them by doing:
var objects = scene.drillPick(new Cesium.Cartesian2(event.clientX, event.clientY));
which returns an array length 0.

I think it may be because I am just adding them to a billboard collection?
I have:
billboards = viewer.cesiumWidget.scene.primitives.add(new Cesium.BillboardCollection());
and then I add them by:
billboards.add ({
position: cartesian,
image: pinBuilder.fromColor(Cesium.Color.RED, 30).toDataURL(),
verticalOrigin: Cesium.VerticalOrigin.BOTTOM

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

I am simply trying to pick a billboard that appears on my globe.

4. The Cesium version you're using, your operating system and browser.

Cesium 1.47, Centos, Firefox

Ok, I think I figured it out! I needed to do event.position, not event.clientX or event.clientY.

So now it is recognizing that it has picked the object, but it is not recognizing that it is in the collection (when I do billboards.contains(object)) or that I have attached a boolean property as suggested in https://groups.google.com/forum/#!topic/cesium-dev/UU5Kd52TIKY

Fixed it by doing object.primitive!

Glad you figured it out! Here’s a complete example for picking billboards in a collection for anyone who’s wondering. (Hover over the billboards to highlight them)