pickEllipsoid issue

So I'm trying to do a thing where you double click on the map and some stuff is kicked off using that position. I seem to be having some trouble getting it to work though. I basically cribbed from the Picking example in the Sandcastle like so. Immediately after creating the Cesium viewer:

var scene = this.viewer.scene;
var ellipsoid = scene.globe.ellipsoid;
var handler = new Cesium.ScreenSpaceEventHandler(scene.canvas);
handler.setInputAction(function(movement) {
     var cartesian = scene.camera.pickEllipsoid(movement.startPosition,ellipsoid);
}, Cesium.ScreenSpaceEventType.LEFT_DOUBLE_CLICK);

And when I try this code, pickEllipsoid throws an exception because it says it expects a windowPosition. This code is almost exactly the same as the Sandcastle example, so I don't know why it's balking at my version and not that. Am I missing something?

Mouse move events have a start and end position, but click events only have one position. Try:

handler.setInputAction(function(event) {
var cartesian = scene.camera.pickEllipsoid(event.position,ellipsoid);
}, Cesium.ScreenSpaceEventType.LEFT_DOUBLE_CLICK);