graphql subscriptions with child nodes seems not working for me

Question

First of all, Subscriptions is working if I have next code structure

schema

type Subscription {
   somethingChanged: IResult
}
type IResult {
   id: String!
}

resolvers

export const resolvers = {
   Query: {
   },
   Mutation: {
   },
   Subscription: {
      somethingChanged: {
         subscribe: () => pubSub.asyncIterator(SOMETHING_CHANGED_TOPIC)
      }
   },
};

The problem became be a problem when I try to make somethingChanged node as a child of another account node, like so:

So next schema and resolvers are not working

type Subscription {
   account(token: String!): IAccountAction
}
type IResult {
   id: String!
}
type IAccountAction {
   somethingChanged: IResult
}

export const resolvers = {
   Query: {
   },
   Mutation: {
   },
   Subscription: {
      account: (root: any, token: string) => ({
         somethingChanged: {
            subscribe: () => pubSub.asyncIterator(SOMETHING_CHANGED_TOPIC)
         }
      })
   },
};

ERROR

{ "error": { "message": "Subscription field must return Async Iterable. Received: undefined" } }

I believe that is not an issue and only misunderstanding )) Please help me ))

---

Additional info which might help:

server.ts

import {resolvers} from './generated/resolvers';
import {typeDefs} from './generated/mergedGQLSchemas';
import http from 'http';

const express = require('express');
const { ApolloServer, gql } = require('apollo-server-express');

const PORT = 4001;

const app = express();

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

server.applyMiddleware({
   app
});

const httpServer = http.createServer(app);
server.installSubscriptionHandlers(httpServer);

httpServer.listen(PORT, () => {
  console.log(` Server ready at http://localhost:${PORT}${server.graphqlPath}`);
  console.log(` Subscriptions ready at ws://localhost:${PORT}${server.subscriptionsPath}`);
});