In my opinion the first/initial emission of nullSentinel is missing: ```java ObjectProperty<String> stringProperty = new SimpleObjectProperty<>(); JavaFxObservable.valuesOf(stringProperty, "N/A").subscribe(System.out::println); stringProperty.set("one"); stringProperty.set(null); stringProperty.set("two"); ``` Output: one N/A two I would add "else" block just after "if" at ObservableValueSource:50 ```java if (fxObservable.getValue() != null) { emitter.onNext(fxObservable.getValue()); } else { emitter.onNext(nullSentinel); } ```