I am trying to write some code to send a push notification to a pass on an Apple device using C# .NET and HttpClient over HTTP2 with client certificate authentication. When I run the code, I am seeing the below exception: InnerException = {"Error 12152 calling WinHttpWriteData, 'The server returned an invalid or unrecognized response'."} I am trying to find out why this code is failing? Is it possible to debug/troubleshoot this in some way? Running this code on Windows 10 OS Build version: 21H2 (19044). The application is built on .Net Framework 4.8 and tried using WinHttpHanlder versions 6 and also 7. Also, tried using other third party open source libraries like PushSharp and DotApns. PushSharp does not seem to support Http/2 and dotApns does not support certificate authentication for .net framework. We have no plans to migrate to .net Core. Below is my code: private static string pushToken = "dbc56849<hidden>"; private static string AppleApnServer = "https://api.sandbox.push.apple.com"; public static async Task<PushResult> SendPushNotificationToWalletPass(string notificationContent) { byte[] certificateData = LoadCertificate(); X509Certificate2 certificate = new X509Certificate2(certificateData, String.Empty, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable); string url = $"{AppleApnServer}:443/3/device/{pushToken}"; StringBuilder payload = new StringBuilder(); payload.Append("{ \"aps\" : "); if (string.IsNullOrWhiteSpace(notificationContent)) { payload.Append("\"\" }"); } else { payload.Append(notificationContent); payload.Append(" }"); // close aps dictionary } var handler = new Http2Handler(); handler.ClientCertificates.Add(certificate); using (var httpClient = new HttpClient(handler)) { using (var request = new HttpRequestMessage(HttpMethod.Post, url)) { var messageGuid = Guid.NewGuid().ToString(); request.Content = new StringContent(payload.ToString()); request.Headers.Add("apns-id", messageGuid); request.Headers.Add("apns-push-type", "alert"); using (var response = await httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead)) { HttpStatusCode statusCode = response.StatusCode; string reasonPhrase = response.ReasonPhrase; bool success = response.IsSuccessStatusCode; Console.WriteLine($"APN {(success ? "Delivered Successfully!" : $"Failed to Send! :: StatusCode [{statusCode}] Reason [{reasonPhrase}]")} :: PushToken [{pushToken}]"); if (!success) { switch (statusCode) { case HttpStatusCode.Gone: // The device token is no longer active for the topic. return PushResult.DeviceNotRegistered; default: return PushResult.Failure; } } return PushResult.Success; } } } } public enum PushResult { Success = 0, Failure = 100, DeviceNotRegistered = 200 } // Apple APNS requires http2 but .Net Framework does not support http2 (only built in support in .net core) // found this workaround https://stackoverflow.com/questions/32685151/how-to-make-the-net-httpclient-use-http-2-0/43101990#43101990 private class Http2Handler : WinHttpHandler { protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) { request.Version = new Version("2.0"); return base.SendAsync(request, cancellationToken); } }