CesiumJS Bug

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Hello World!</title>
  <script src="Cesium/Cesium.js"></script>
  <style>
      @import url(Cesium/Widgets/widgets.css);

      #cesiumContainer {
          position: absolute;
          top: 0;
          left: 0;
          height: 100%;
          width: 100%;
          margin: 0;
          overflow: hidden;
          padding: 0;
          font-family: sans-serif;
      }

      body {
          padding: 0;
          margin: 0;
          overflow: hidden;
      }
  </style>
</head>
<body>
  <div id="cesiumContainer"></div>
  <script>
    var cesiumWidget = new Cesium.CesiumWidget('cesiumContainer');

    var terrainProvider = new Cesium.CesiumTerrainProvider({
      url : 'http://cesiumjs.org/smallterrain'
    });
    cesiumWidget.centralBody.terrainProvider = terrainProvider;

    // *** ADD A MODEL TO THE SCENE ***
    var ellipsoid = cesiumWidget.centralBody.getEllipsoid();
    var rotateX = Cesium.Matrix4.fromRotationTranslation(Cesium.Matrix3.fromRotationX(Cesium.Math.toRadians(90.0)), Cesium.Cartesian3.ZERO);
    var modelMatrix = Cesium.Matrix4.multiply(Cesium.Transforms.eastNorthUpToFixedFrame(ellipsoid.cartographicToCartesian(Cesium.Cartographic.fromDegrees(14.281782, 40.858374, 0.0))), rotateX);

    var scene = cesiumWidget.scene;

    var model = scene.getPrimitives().add(Cesium.Model.fromGltf({
        url : './duck/duck.json',
        modelMatrix : modelMatrix,
        scale : 1.0,
        debugWireframe : false,
        allowPicking : false,
        debugShowBoundingVolume : false
    }));

    model.readyToRender.addEventListener(function(model) {

        statistics = Cesium.gltfStatistics(model.gltf);
        console.log(statistics);

        var worldBoundingSphere = model.computeWorldBoundingSphere();
        var center = worldBoundingSphere.center;
        var transform = Cesium.Transforms.eastNorthUpToFixedFrame(center);

        // View in east-north-up frame
        var camera = scene.getCamera();
        camera.transform = transform;
        camera.controller.constrainedAxis = Cesium.Cartesian3.UNIT_Z;

        var controller = scene.getScreenSpaceCameraController();
        controller.setEllipsoid(Cesium.Ellipsoid.UNIT_SPHERE);
        controller.enableTilt = false;

        // Zoom in
        camera.controller.lookAt(
            new Cesium.Cartesian3(0.0, -worldBoundingSphere.radius * 0.25, worldBoundingSphere.radius * 2.0),
            Cesium.Cartesian3.ZERO,
            Cesium.Cartesian3.UNIT_Z);
    });

  </script>
</body>
</html>