What are the correct min and max values for Gluon maps zooming?

Question

I am trying out Gluon maps. As I understand it, Gluon maps uses OpenStreetMap which has a zoom range of 0 to 20 (but this value is not mentioned in the MapView javadoc). I create this code:

public class MapTest2 extends Application
{
    MapView mapView = new MapView();
    MapPoint mapPoint = new MapPoint(51.509865d,-0.118092d);    // London, England
        
    @Override
    public void start(Stage stage) throws Exception {
        StackPane sp = new StackPane();
        sp.getChildren().add(mapView);
        
        mapView.setCenter(mapPoint);
        mapView.setZoom(42.8);          // ? and -1.1 works but -1.3 fails
        
        Scene  scene = new Scene(sp, 500, 300);
        stage.setScene(scene);
        stage.show();
    }
    
    public static void main(String[] args) {
        MapTest2.launch(MapTest2.class, args);
    }
}

where I can set the positive zoom value far higher than 20. Some considerable zooming out on the mouse wheel returns me to a reasonable view. A zoom value of -1.1 works but -1.3 fails with:

Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
:
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index -2 out of bounds for length 20
    at com.gluonhq.impl.maps.BaseMap.loadTiles(BaseMap.java:347)
    at com.gluonhq.impl.maps.BaseMap.layoutChildren(BaseMap.java:490)
:

What are the real minimum and maximum zoom ranges, and should I use only integer values for zoom as there seems to be some rounding going on?