System.Net.WebException when using HttpClient against Runscope bucket

Question

I'm playing with Runscope using Mono 3 on the Mac. Using the code below I'm getting an exception:

System.Net.WebException: Error getting response stream (Write: The authentication or decryption has failed.): SendFailure ---> System.IO.IOException: The authentication or decryption has failed. ---> Mono.Security.Protocol.Tls.TlsException: The authentication or decryption has failed.

I don't understand why, as the ServicePointManager.ServerCertificateValidationCallback should effectively approve all certificates.

Also note that the code works perfectly under Visual Studio on Windows.

Any ideas?

class Program
    {
        private static Uri url = new Uri("https://api-github-com-[MY_PERSONAL_TOKEN_HERE].runscope.net/");

        static void Main(string[] args)
        {
            ServicePointManager.ServerCertificateValidationCallback +=
                (sender, cert, chain, sslPolicyErrors) => true;
            MakeRequest();
            Console.ReadKey ();
        }

        private static async Task MakeRequest()
        {
            try
            {
            var httpClient = new HttpClient();
            var response = await httpClient.GetAsync(url);
            var body = await response.Content.ReadAsStringAsync();
            Console.WriteLine("Body: " + body);
            }
            catch(Exception ex)
            {
                Console.WriteLine (ex);
            }
        }
    }