Convert event without EventArgs using Observable.FromEvent

I'm struggling with converting the following event to an IObservable:

public delegate void _dispSolutionEvents_OpenedEventHandler();
event _dispSolutionEvents_OpenedEventHandler Opened;

The event comes from a library so I can't change it. The overload of IObservable.FromEvent that should do it has the following signature:

public static IObservable<Unit> FromEvent
    ( Action<Action> addHandler
    , Action<Action> removeHandler
    )

So I tried converting the event like this:

var opened = Observable.FromEvent
    ( h => _SolutionEvents.Opened += h
    , h => _SolutionEvents.Opened -= h
    );

But the compiler doesn't like _SolutionEvents.Opened += h and _SolutionEvents.Opened += h because

Cannot implicitly convert type 'System.Action' to 'EnvDTE._dispSolutionEvents_OpenedEventHandler'.

I don't think that I can just say_SolutionEvents.Opened += new _dispSolutionEvents_OpenedEventHandler(h) because then removal won't work because I have a different instance, right?

There is another overload of Observable.FromEvent with the following signature:

public static IObservable<TEventArgs> FromEvent<TDelegate, TEventArgs>
    ( Func<Action<TEventArgs>, TDelegate> conversion
    , Action<TDelegate> addHandler
    , Action<TDelegate> removeHandler
    )

This one allows to convert the action to an event handler, but it seems to only work with TEventArgs.

Is Rx missing an appropriate overload or am I missing something?