Unable to subscribe to nested subscriptions

[wdkr]

Product

Hot Chocolate

Version

15.1.7

Link to minimal reproduction

https://github.com/wdkr/HotChocolateDemo

Steps to reproduce

I can organize my queries by grouping them into nested classes. For example:

public class Query
{
    public LibraryQueries Library() => new();

    public class LibraryQueries
    {
        public Book GetBook()
        {
            return new Book { Title = "C# in depth", Author = "Jon Skeet" };
        }
    }
}

I can then run the following query:

query {
  library {
    book {
      title
      author
    }
  }
}

This works fine. If I try to do the same with subscriptions:

public class Subscription
{
    public LibrarySubscriptions Library() => new();

    public class LibrarySubscriptions
    {
        [Subscribe]
        [Topic("BookAdded")]
        public Book BookAdded([EventMessage] Book book) => book;
    }
}

And try to subscribe to the bookAdded event:

subscription {
  library {
    bookAdded {
      title
      author
    }
  }
}

I get the following websocket error:

You must declare a subscribe resolver for subscription fields.

Is this supposed to work or unsupported?

What is expected?

Being able to subscribe to subscriptions defined in nested classes.

What is actually happening?

The websocket connection fails and the error message "You must declare a subscribe resolver for subscription fields." is shown in the message log.

Relevant log output

Additional context

No response