List objects in Webi reports

I'm using powershell to query the RESTful service on BI 4.2 SP3.
My objective is to get a list of Universe Objects used in webi reports

With this query:
http://<server-name>:6405/biprws/v1/documents
I can get the cuid, name, description, id, type, ownerid, parentid, and updated date for all the reports

(Note that http://<server-name>:6405/biprws/raylight/v1/documents returns nothing.)

How do I use the SI_ID from each report to get the objects it uses through the RESTful Web Services SDK?

I looked at this thread: https://archive.sap.com/discussions/thread/3945568

So, I try:
http://<server-name>:6405/biprws/raylight/v1/documents/{SI_ID-of-a-Webi-document}/dataproviders

(Note that I the same result whether or not 'raylight' is in the URL.)

All I get back is: @{dataproviders=}

Your insight would be greatly appreciated.

Thanks!