Can't add resource category to package with vRO 7.1 REST API

I am trying to add a Resource Element category to a package in Orchestrator 7.1, using the REST API.

The call:

/vco/api/packages/{packageName}/resource_category/System%20Config

always fails with the message that the given folder name doesn't exist:

{"status":404,"message":"Resource category with name System Config doesn't exist"}

I can’t get the call to find anything but top level resource categories. So, it works for “Library” but not for any other “folder” below that top level?!?!

Any tips appreciated.

Thanks,

Ed