Android : Retrofit chaining observables

I'm trying to use retrofit with rxjava. I have a problem chaining retrofit observables with one another or with observables created by me. Some example:

    Observable<List<Friend>> friendsListObservable = friendsService.getFriends();
    Observable<Void> updateReqestObservable = friendsListObservable.switchMap(friends -> {
        Log.d(TAG, "Hello");
        return userAPI.updateFriends(session.getUserId(), friends);
    }).subscribe();

Everything gets called until it gets to switchMap. So hello is never displayed, but if I return for instance Observable.just(null) instead of the retrofit observable it works fine. Also if I user the retrofit observable without chaining, it works.