The index of the underlying data on the parent property

I have an abstract Entity event and a sub-object of this SpecificEvent. The event has a created_at property. SpecificEvent has additional properties

I am trying to create a composite index on a SpecificEvent including created_at (parent super property), but I am getting an error, compile the data model:

UserDataModel.xcdatamodeld / UserDataModel.xcdatamodel: in the Compound Indexes attribute for the SpecificEvent object, created_at is not a valid property

It looks like xcode cannot find the parent property because all other properties are working. Is there a way to fix this or is this just a limitation? I looked through the docs but couldn't find anything on indexing parent properties and very little on complex indexing in general.