OWIN simple startup class

The OWIN startup class below generates the following error on ubuntu 18.04 xsp4 on .net 4.6 :

Application Exception
System.Web.HttpException
The resource cannot be found.

Description: HTTP 404.The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.

Details: Requested URL: /
Exception stack trace:
  at System.Web.StaticFileHandler.ProcessRequest (System.Web.HttpContext context) [0x00094] in /build/mono-5.18.1.3/mcs/class/System.Web/System.Web/StaticFileHandler.cs:77
  at System.Web.DefaultHttpHandler.BeginProcessRequest (System.Web.HttpContext context, System.AsyncCallback callback, System.Object state) [0x0007f] in /build/mono-5.18.1.3/mcs/class/System.Web/System.Web/DefaultHttpHandler.cs:101
  at System.Web.HttpApplication+<Pipeline>d__225.MoveNext () [0x008d4] in /build/mono-5.18.1.3/mcs/class/System.Web/System.Web/HttpApplication.cs:1335
  at System.Web.HttpApplication.Tick () [0x00000] in /build/mono-5.18.1.3/mcs/class/System.Web/System.Web/HttpApplication.cs:927
Version Information: 5.18.1.3 (tarball Tue Apr 9 16:16:30 UTC 2019); ASP.NET Version: 4.0.30319.42000



using System;
using Microsoft.Owin;
using Microsoft.Owin.Diagnostics;
using Owin;

[assembly: OwinStartup(typeof(KatanIIS.Startup))]

namespace KatanIIS
{
    public class Startup
    {

        public void Configuration(IAppBuilder app)
        {


            app.Run(context =>
            {
               context.Response.ContentType = "text/plain";
                //context.Response.Write("test");
                return context.Response.WriteAsync("Hello");
            });

        }
    }
}