Not sure why but seems the parameter "defaultHandler" is useless here

[4engkai]

https://github.com/aspnet/Routing/blob/dev/src/Microsoft.AspNetCore.Routing/RouteBuilder.cs

public RouteBuilder(IApplicationBuilder applicationBuilder, IRouter defaultHandler)
{
    if (applicationBuilder == null)
    {
        throw new ArgumentNullException(nameof(applicationBuilder));
    }

    if (applicationBuilder.ApplicationServices.GetService(typeof(RoutingMarkerService)) == null)
    {
        throw new InvalidOperationException(Resources.FormatUnableToFindServices(
            nameof(IServiceCollection),
            nameof(RoutingServiceCollectionExtensions.AddRouting),
            "ConfigureServices(...)"));
    }

    ApplicationBuilder = applicationBuilder;
    ServiceProvider = applicationBuilder.ApplicationServices;

    Routes = new List<IRouter>();
}

[Eilon]

Ah, that looks like a bug! I think it just needs to set the DefaultHandler property to that value.