Join two subgraphs using Graphql Fusion

Question

I am trying to get result when I query like below in fusion gateway, basically join of Product and Review like for each product get the review details for each product,I know Fusion is in .Net but I thinks as long as introspection work it can work with spring boot graphql service also, so I tried to use @lookup for my use case running query like this way :

 query Products {
 products {
 id
 name
 reviews {
 comment
 productId
 rating
 }
 }
 }

individual subgraph data fetching is working but unable to run above query, getting below error 
{

 "errors": [
 {
 "message": "Unexpected Execution Error"
 }
 ]
 }

-----------------------------------------------------
product subgraph schema.graphqls

 directive @lookup(by: String) on FIELD_DEFINITION
 
 type Product
 {
 id: ID!
 name: String
 }
 
 type Query
 {
 products: [Product]
 productById(id: ID!): Product @lookup(by: "id")
 }

-----------------------------
review subgraph schema.graphqls

directive @lookup(by: String) on FIELD_DEFINITION
 
 type Review
 {
 productId: ID!
 rating: Int
 comment: String
 }
 
 type Query
 {
 reviews: [Review]
 reviewsByProductId(productId: ID!): [Review!]! @lookup(by: "productId")
 }
 
 type Product {
 id: ID!
 } 
 extend type Product {
 reviews: [Review!]! @lookup(by: "productId")
 }

both the subgraphs I am running in spring boot graphql and merging these subgraphs using fusion gateway

tried @lookup in schema to make the join and byId resolver

I was expecting to get

{
 "data": {
 "products": [
 {
 "id": "1",
 "name": "Product A",
 "reviews": [
 { "comment": "Excellent", "productId": "1", "rating": 5 },
 { "comment": "Very Good", "productId": "1", "rating": 4 }
 ]
 },
 {
 "id": "2",
 "name": "Product B",
 "reviews": [
 { "comment": "Average", "productId": "2", "rating": 3 }
 ]
 }
 ]
 }
}