ArcGisImageServerTerrainProvider problem

CesiumJS
1

var viewer = new Cesium.Viewer('cesiumContainer');

var terrainProvider = new Cesium.ArcGisImageServerTerrainProvider({
  url : '//server.arcgisonline.com/arcgis/rest/services/World_Terrain_Base/MapServer'
});
viewer.terrainProvider = terrainProvider;

this url:
server.arcgisonline.com/arcgis/rest/services/World_Terrain_Base/MapServer
is accessable without token proxy.
But when running,it raises the following error and cannot load the terrain into the global surface.

An error occurred in "ArcGisImageServerTerrainProvider": Failed to obtain terrain tile X: 1 Y: 0 Level: 0.
An error occurred in "ArcGisImageServerTerrainProvider": Failed to obtain terrain tile X: 0 Y: 0 Level: 0.

Use cesium-1.6 sandcastle