ctx.pubsub.asyncIterator is not a function when running orm/graphql-subscriptions

[Jaime02]

Hello, I am trying to run the orm/graphql-subscriptions example. I have followed the steps described in the README file. When I open GraphQL playground and execute the suggested subscription:

subscription {
  newPost {
    id
    title
    published
    author {
      id
      email
      name
    }
  }
}

I get the following error:

{
  "errors": [
    {
      "message": "ctx.pubsub.asyncIterator is not a function",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "newPost"
      ]
    }
  ]
}