Unable to create a wall entity with negative heights.

I want to create a wall entity which is according to the terrain heights.

so I’m creating wall entity taking the heights from the terrain.But when the heights are negative I’m not able to create a wall entity.I’m not even getting an error.

How to solve this.Sandcastle link of the example i have tried.

Underground visualization isn’t currently fully supported, but it’s something on our radar, see: https://github.com/AnalyticalGraphicsInc/cesium/issues/5665