@provides on interface errors out

Help
1

I have two subgraphs: Sub1, Sub2, and Gateway.

Sub1 implements:

type Category {
  id: ID!
  name: String!
  # and many other fields
}

I want to add Sub2 that provides a part of Category:

extend type Category @key(fields: "id") {
  id: ID! @external
  name: String! @external
}

interface I {
  category: Category! @provides(fields: "name")
}

type B implements I {
  category: Category! @provides(fields: "name")
}
# there are many other types that implement I

union Sub2Data = A | B | C;

type Sub2Response {
  count: Int!
  data: [Sub2Data!]!
}

type Query {
  sub2Query: Sub2Response!
}

I want to query:

query Sub2Query {
  sub2Query {
    count
    data {
       ... on I {
          category {
             name
          }
       }
   }
}

When I send the query to Gateway, the Gateway sends the _entities query to Sub1:

query GW_Test($representations: [_Any!]!) {
  _entities(representations: $representations) {
    ... on Category {
        name
    }
  }
}

And, Sub1 errors out with

Error: Cannot return null for non-nullable field Category.name.

When I send the same query (sub2Query) to Sub2 directly, it’s successful.

How can I make Category.name to come from Sub2 when I’m querying sub2Query?

Thanks.
Sam

Edit:
When I put category field out of the interface to concrete types, it works:

What works:

interface I {}
type B implements I {
  category: Category! @provides(fields: "name")
}

And change the query to:

query Sub2Query {
  sub2Query {
    count
    data {
       ... on A {
          category {
             name
           }
        }
       ... on B {
          category {
             name
           }
        }
       ... on C {
          category {
             name
           }
        } 
   }
}

This makes query cumbersome. Is there way to make @provides work on interface?