ArcGIS ImageryProvider; feature selection crashes Cesium

We’re pointing to an ArcGIS map server that uses a single layer which I specify in the layer section. However, when clicking anywhere on the globe that does not contain an entity, Cesium crashes with an error, “i is undefined in cssFromColorString”.
This behavior does happen on the public ArcGIS map server referenced in the Cesium examples. Does anyone have any thoughts on why this might be occurring?
provider = new Cesium.ArcGisMapServerImageryProvider({
url : _mapServerUrl,
layer: ‘basemap’
});
In addition, ‘baseLayerPicker’ is turned off in our viewer, as below. Would it be better to add the imagery provider as a layer rather than the base provider?
var options = {
animation: false,
timeline: false,
homeButton: true,
navigationHelpButton: true,
fullscreenButton: false,
geocoder: false,
infoBox: true,
baseLayerPicker: false,
imageryProvider: imageryProvider,
fullscreenElement: false,
navigationInstructionsInitiallyVisible: false,
automaticallyTrackDataSourceClocks: false,
targetFrameRate: 18,
sceneMode: Cesium.SceneMode.SCENE3D,
mapProjection: new Cesium.WebMercatorProjection(),
contextOptions: {
allowTextureFilterAnisotropic : false
}
};
In the above example, imageryProvider: imageryProvider is the same provider as specified in the first code snippet.

Other notes: Cesium 1.22.1

Thanks.

Correction in the above: "This behavior does NOT happen on the public ArcGIS map server referenced in the Cesium examples. Does anyone have any thoughts on why this might be
occurring?