Incorrect height when using scene.globe.getHeight(long,lat)

I tried to query the height using a combination of Cesium.createWorldTerrain() and viewer.scene.globe.getHeight(Cesium.Cartographic.fromDegrees(longitudeString, latitudeString));

However, I get the height/altitude of San Francisco airport (SFO) a negative number (-29.3 meters).
Does anyone know why?

Please see the code that I put on
SandCastle Run

code snippet:

const viewer = new Cesium.Viewer('cesiumContainer', {
  terrainProvider: Cesium.createWorldTerrain()

// more javascript code here …

   var altitudeString = 
        scene.globe.getHeight(Cesium.Cartographic.fromDegrees(Number(longitudeString), Number(latitudeString)));

Thanks in advance for your help!

Not sure what getHeight() is connected to (possibly sampleHeight / sampleTerrain, but then both of those depends on how close to picked object you are), but have you tried sampleTerrainMostDetailed()? It should use the heights accuracy loaded; Global - Cesium Documentation