camera go down under the ground

I have some question when using Cesium to render Terrain. The camera will go down under the ground and sometime throw an exception, which stops the rendering. The image show the problem when occured. How can I solve
the problem. I wish forbid the camera to go down under the ground.

The Cesium version is 1.47, win10 system, chrome

Looks like this is a popular feature request for Cesium that just hasn’t been implemented yet. You can follow the discussion/progress on it here:

We would also be interested in this feature/fix. We see the error like “cannot read property west of undefined” sometimes if the user zooms to an odd location.