GraphQL Subscriptions return an empty (null) response

Question

I have an issue with a very simple GRAPHQL server trying to make subscriptions work. Using pubsub to subscribe and publish an event when a user signs in.

On signing in, the mutation completes successfully, but the corresponding subscription returns "null"

See below screenshots from GraphQL Playground:

Pubsub instantiation:

const { PubSub } = require("apollo-server");

const pubsub = new PubSub();
const USER_SIGNED_IN = "USER_SIGNED_IN";

typedefs:

  type User {
    id: ID!
    firstName: String!
    lastName: String
    email: String!
    cell: String
  }

  type AuthPayload {
    token: String
    user: User
  }

  type Subscription {
    userAdded: User
  }

resolver-mutation:

signin: async (parent, args, context) => {
  const user = users.find(user => user.email === args.email);
  if (!user) {
    throw new Error("No account found.  Please sign up.");
  }
  const valid = await bcrypt.compare(args.password, user.password);
  if (!valid) {
    throw new Error("Invalid Password");
  }
  const token = jwt.sign(
    {
      userId: user.id
    },
    "secret!"
  );
  pubsub.publish(USER_SIGNED_IN, { user });
  return {
    user,
    token
  };
}

resolver-subscription:

userAdded: {
      subscribe: () => pubsub.asyncIterator(USER_SIGNED_IN)
    }

Code is available on CodeSandBox (Seems like subscription times out quickly when using codesandbox. I recommend you download the code and npm install the npm start to test): CodeSandBox

Appreciate any assistance.