How do I make GraphQL subscriptions in NodeJS via Apollo Client?

I want to make a subscription to a GraphQL server. The application is running in a NodeJS script (i.e. not in the webbrowser).

Here is what I currently do:

const fetch = require("node-fetch").default;
const apollo = require("apollo-boost");
const ApolloClient = apollo.default;
const { gql } = require("apollo-server");

const apolloClient = new ApolloClient({
  uri: "http://localhost:4000/graphql",
  fetch
});

apolloClient.subscribe({
  query: gql`
    subscription {
      startTaskRequested {
        pkRobot
        taskName
      }
    }
  `,
}).subscribe({
  next(x) { console.log(x) },
  error(err) { console.log(`Finished with error: ${ err }`) },
  complete() { console.log('Finished') }
});

The resulting ouput is:

{ data: { startTaskRequested: null } }
Finished

On the GraphQL Server, I can see that the corresponding resolver is never called.

If I make the same subscription query using Apollo's Playground, the subscription works and I get the results that I expect: Apollo Playground

I bumped my head against this for many hours already and I would greatly appreciate it if someone could point me in the right direction.