Bug? - Feature Service Symbolized with Null Value

I've been trying to create map service with feature access that is symbolized on unique values. The values are grouped into broader categories. I've found that if one of my categories contains null values, then Geocortex will not let you add the Feature service to a site. If I remove the null value from that symbol group (in ArcGIS), and just leave the All other values category enabled, then Geocortex will accept the feature service. 

The error you get when trying to add the feature service is:

Value cannot be null. Parameter name: key

 

The problem seems specific to Geocortex, as ArcGIS Online will accept the Feature Service in either scenario.

 

Thanks,

 

Jody