Cesium crashes when changing label height reference

[hpinkos]

var viewer = new Cesium.Viewer('cesiumContainer', {
    baseLayerPicker: false,
    terrainProvider: new Cesium.CesiumTerrainProvider({
        url : 'https://assets.agi.com/stk-terrain/world'
    })
});

var labelCollection = viewer.scene.primitives.add(new Cesium.LabelCollection({
    scene : viewer.scene
}));

var label = labelCollection.add({
    position : Cesium.Cartesian3.fromDegrees(-110, 50),
    text: 'text',
    heightReference: Cesium.HeightReference.CLAMP_TO_GROUND
});

var clamp = true;
Sandcastle.addToolbarButton('Toggle Height Reference', function() {
    clamp = !clamp;
    var heightReference = clamp ? Cesium.HeightReference.CLAMP_TO_GROUND : Cesium.HeightReference.NONE;
    label.heightReference = heightReference;
});