Missing optional routing argument not null

Hi!

I have a simple route with an optional "version" argument:

routes.MapRoute(name: "pages", template: "{action=Index}/{version?}", defaults: new { Controller = "Pages" });

and corresponding action method:

public IActionResult Index(Version version = null) { ... }

However, when I navigate to / the action method gets invoked with version = 0.0, not null. Where does that value come from and how can I fix that?

Thanks!