source.Delay(observable) throws NullReferenceException if source emits a value immediately

[christopherpaul]

The following code throws a NullReferenceException:

            var source = Observable.Return(1);
            var delayed = source.Delay(_ => Observable.Return(2));
            delayed.Subscribe(Console.Write); // NullReferenceException

This only seems to happen if source emits a value immediately on subscription.
The kind of observable used in the argument to Delay does not seem to make any difference.

System.Reactive v4.1.6
.NET Framework 4.7.2

[clairernovotny]

Any chance you can add a failing unit test with this using the virtual time scheduler?