Displaying extruded polygons while using GroundPrimitive

The goal of my little side project is to show buildings (extruded polygons) draped over a terrain.

I have been able to add a terrain mesh, a geoJSON of the "building footprints" to the map. Screenshot here: http://imgur.com/TocZDMT

Now I want to extrude the polyons. following is my code to generate the extrusions

var allGeometryInstances = ;
// loop over the polygon features
// create a degreesArray for the polygon feature geojson
var degreesArray = coordsToDegreesArray(curFeat['geometry']['coordinates'][0])
var extrudedPolygon = new Cesium.PolygonGeometry({
                polygonHierarchy : new Cesium.PolygonHierarchy(
                extrudedHeight: 40

       var polygonInstance = new Cesium.GeometryInstance({
           geometry : extrudedPolygon,
           id : randomid(),
               color: new Cesium.ColorGeometryInstanceAttribute(0.0, 1.0, 1.0, 0.5)


and finally:
    scene.primitives.add(new Cesium.GroundPrimitive({
            geometryInstances : allGeometryInstances

When I run this code, the height is not rendered. What am I missing?


OK I reveiwed the archive and looks like what I am trying to do is not supported. I am going to try to sample the underlying terrain for polygon heights and draw polygon via that instead of using the groundprimitive.