Map projection and nested type

Yes, I tried that. I am getting the same error:

Expected to find a node at '  client@891' but found Map instead

for the query:

 downline(login: "c4" ) {
   id
   name
   sponsor {
    id
    name
  }
 }
 }

I also find the very similar question posted here, without an answer:

Any other idea? Is this doable at all?
Thanks!