runtime: WinHttpException on WebRequest

The following snippet of code call a web service using a certificate from the user store. I got a generic security exception

System.Net.Http.WinHttpException (0x80072F8F): A security error occurred

with NativeErrorCode 12175. The same snippet works against .NET Framework 4.6.1.

Any guidance on troubleshooting this issue?

using (var store = new X509Store(StoreName.My, StoreLocation.CurrentUser))
{
    store.Open(OpenFlags.ReadOnly);
    var cert = store.Certificates.Find(X509FindType.FindBySerialNumber, "serial_number", false);
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://url");
    request.ClientCertificates.Add(cert[0]);
    request.Method = "GET";
    request.ServerCertificateValidationCallback = delegate
    {
        // I know this is unsafe
        return true;
    };
    HttpWebResponse response = (HttpWebResponse)request.GetResponse();

    var encoding = Encoding.UTF8;
    using (var reader = new StreamReader(response.GetResponseStream(), encoding))
    {
        string responseText = reader.ReadToEnd();
    }
}

[EDIT] C# syntax highlighting by @karelz