Even more feedback...

Author: JunTaoLuo

src/Microsoft.AspNet.Hosting/WebHostBuilder.cs

@@ -187,6 +187,11 @@ public WebHostBuilder UseServerFactory(string assemblyName)
 
         public WebHostBuilder UseServerFactory(IServerFactory factory)
         {
+            if (factory == null)
+            {
+                throw new ArgumentNullException(nameof(factory));
+            }
+
             _serverFactory = factory;
             return this;
         }

src/Microsoft.AspNet.TestHost/ClientHandler.cs

@@ -130,7 +130,7 @@ internal RequestState(HttpRequestMessage request, PathString pathBase, IHttpCont
                     request.Headers.Host = request.RequestUri.GetComponents(UriComponents.HostAndPort, UriFormat.UriEscaped);
                 }
 
-                HttpContext = factory.Create(new FeatureCollection());
+                HttpContext = _factory.Create(new FeatureCollection());
 
                 HttpContext.Features.Set<IHttpRequestFeature>(new RequestFeature());
                 _responseFeature = new ResponseFeature();