JS API - Viewshed analysis sample is broken

[Neocyte]

Hello JS API team,

Clicking anywhere on the viewshed analysis sample sometimes returns an uncaught promise error. You can replicate this by clicking on the North/South Pole. Additionally, several tiles fail to load. 

This is on version 4.18 of the ArcGIS Javascript API:
https://developers.arcgis.com/javascript/latest/sample-code/geoprocessing-viewshed/

It seems that when the geoprocessor executes the parameters and promises to draw the result data, one of the parameters doesn't get passed...

var params = {
  Input_Observation_Point: featureSet,
  Viewshed_Distance: vsDistance
};
gp.execute(params).then(drawResultData);