Mount Hasura on Apollo federated gateway

shim.js

const { ApolloServer } = require("apollo-server");
const gql = require("graphql-tag");
const fetch = require("node-fetch");
const {
  introspectionQuery,
  buildClientSchema,
  printSchema
} = require("graphql");

const typeDefs = gql`

  schema {
    query: query_root
  }

  type _Service {
    sdl: String
  }
  
  type query_root {
    _service: _Service!
  }

`;

const hasuraURL = "https://somehasuraurl.com/v1/graphql";

async function getHasuraSchema() {
  return await fetch(hasuraURL, {
    method: "POST",
    headers: {
      Accept: "application/json",
      "Content-Type": "application/json"
    },
    body: JSON.stringify({
      query: introspectionQuery
    })
  })
    .then(res => res.json())
    .then(introspectionJSON => {
      delete introspectionJSON.data.__schema.subscriptionType; // apollo-gateway does not support subscriptions as of now and having subscription type throws a wierd error!
      return printSchema(buildClientSchema(introspectionJSON.data));
    });
}

const resolvers = {
  query_root: {
    _service: async () => {
      var hasuraSchema = await getHasuraSchema();
      return { sdl: hasuraSchema };
    }
  }
};

const schema = new ApolloServer({ typeDefs, resolvers });

schema.listen({ port: process.env.PORT }).then(({ url }) => {
  console.log(`schema ready at ${url}`);
});

I get this error
Can you give me a hint to work it out?

(node:23754) UnhandledPromiseRejectionWarning: Error: A valid schema couldn't be composed. The following composition errors were found:
[hasura] Mutation -> Found invalid use of default root operation name Mutation. Mutation is disallowed when Schema.mutation is set to a type other than Mutation.
[hasura] Query -> Found invalid use of default root operation name Query. Query is disallowed when Schema.query is set to a type other than Query.
at ApolloGateway.createSchemaFromServiceList (/Users/hbkimz/Documents/works/capa/node_modules/@nestjs/graphql/node_modules/@apollo/gateway/dist/index.js:432:19)
at ApolloGateway.updateByComposition (/Users/hbkimz/Documents/works/capa/node_modules/@nestjs/graphql/node_modules/@apollo/gateway/dist/index.js:303:48)
at processTicksAndRejections (internal/process/task_queues.js:93:5)
at async ApolloGateway.updateSchema (/Users/hbkimz/Documents/works/capa/node_modules/@nestjs/graphql/node_modules/@apollo/gateway/dist/index.js:279:13)
at async ApolloGateway.loadDynamic (/Users/hbkimz/Documents/works/capa/node_modules/@nestjs/graphql/node_modules/@apollo/gateway/dist/index.js:258:13)
at async ApolloGateway.load (/Users/hbkimz/Documents/works/capa/node_modules/@nestjs/graphql/node_modules/@apollo/gateway/dist/index.js:229:15)
(Use node --trace-warnings ... to show where the warning was created)
(node:23754) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag --unhandled-rejections=strict (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:23754) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.