request.GetRequestStream() throws an “Value cannot be null” exception

I have a function to do http request to a api servers who accept jsons:

 private static string DoRequest(object objToSend, string Url)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
            request.Method = "POST";
            request.ContentType = "application/json";
            request.Accept = "application/json";
            var jsonString= Obj2Json(objToSend);
            if (string.IsNullOrEmpty(jsonString))
                throw new ArgumentNullException("objToSend", "Objcet was converted to json string and produces an empty string");
            var buffer = Encoding.UTF8.GetBytes(jsonString);
            request.ContentLength = buffer.Length;
            using (var stream = request.GetRequestStream())
            {
                stream.Write(buffer, 0, buffer.Length);
            }
            var response = (HttpWebResponse)request.GetResponse();
            var sResponse = new System.IO.StreamReader(response.GetResponseStream()).ReadToEnd();

            return sResponse;
        }

When I run the line:

using (var stream = request.GetRequestStream())

I got an Exception: 'System.ArgumentNullException' in mscorlib.dll ("Value cannot be null.") System.ArgumentNullException.

in the StackTrace there was one line that says:

"System.Enum.TryParseEnum(Type enumType, String value, Boolean ignoreCase, EnumResult& parseResult)"

the Exception is disappearing by clicking the F10 key.

Does anyone have any idea about, why the exception was thrown?

I want to emphasize that it throws the exception at the GetRequestStream(), that is before to any use of the stream.Write()

After wasting huge of time, I discovered that the excaption was thrown from ServicePointManager.cs that is part of system.dll. Here is the function was causes the exception:

private static void LoadDisableStrongCryptoConfiguration()
{
    try
    {
        bool disableStrongCryptoInternal = false;
        int schUseStrongCryptoKeyValue = 0;

        if (LocalAppContextSwitches.DontEnableSchUseStrongCrypto)
        {
            //.Net 4.5.2 and below will default to false unless the registry key is specifically set to 1.
            schUseStrongCryptoKeyValue =
                RegistryConfiguration.GlobalConfigReadInt(strongCryptoValueName, 0);

            disableStrongCryptoInternal = schUseStrongCryptoKeyValue != 1;
        }
        else
        {
            // .Net 4.6 and above will default to true unless the registry key is specifically set to 0.
            schUseStrongCryptoKeyValue =
                RegistryConfiguration.GlobalConfigReadInt(strongCryptoValueName, 1);

            disableStrongCryptoInternal = schUseStrongCryptoKeyValue == 0;
        }

        if (disableStrongCryptoInternal)
        {
            // Revert the SecurityProtocol selection to the legacy combination.
            s_SecurityProtocolType = SecurityProtocolType.Tls | SecurityProtocolType.Ssl3;
        }
        else
        {
            s_SecurityProtocolType =
                SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;

            string appSetting = RegistryConfiguration.AppConfigReadString(secureProtocolAppSetting, null);

            SecurityProtocolType value;
            try
            {
                value = (SecurityProtocolType)Enum.Parse(typeof(SecurityProtocolType), appSetting);
                ValidateSecurityProtocol(value);
                s_SecurityProtocolType = value;
            }
            // Ignore all potential exceptions caused by Enum.Parse.
            catch (ArgumentNullException) { }
            catch (ArgumentException) { }
            catch (NotSupportedException) { }
            catch (OverflowException) { }
        }

        disableStrongCrypto = disableStrongCryptoInternal;
    }
    catch (Exception e)
    {
        if (e is ThreadAbortException || e is StackOverflowException || e is OutOfMemoryException)
        {
            throw;
        }
    }
}

here is the full call Stack

mscorlib.dll!System.Enum.TryParseEnum(System.Type enumType, string value, bool ignoreCase, ref System.Enum.EnumResult parseResult) mscorlib.dll!System.Enum.Parse(System.Type enumType, string value, bool ignoreCase) System.dll!System.Net.ServicePointManager.LoadDisableStrongCryptoConfiguration() System.dll!System.Net.ServicePointManager.EnsureConfigurationLoaded() System.dll!System.Net.ServicePointManager.SecurityProtocol.get() System.dll!System.Net.TlsStream.ProcessAuthentication(System.Net.LazyAsyncResult result) System.dll!System.Net.TlsStream.Write(byte[] buffer, int offset, int size) System.dll!System.Net.PooledStream.Write(byte[] buffer, int offset, int size) System.dll!System.Net.ConnectStream.WriteHeaders(bool async) System.dll!System.Net.HttpWebRequest.EndSubmitRequest() System.dll!System.Net.HttpWebRequest.SetRequestSubmitDone(System.Net.ConnectStream submitStream) System.dll!System.Net.Connection.CompleteConnection(bool async, System.Net.HttpWebRequest request) System.dll!System.Net.Connection.CompleteStartConnection(bool async, System.Net.HttpWebRequest httpWebRequest) System.dll!System.Net.Connection.CompleteStartRequest(bool onSubmitThread, System.Net.HttpWebRequest request, System.Net.TriState needReConnect) System.dll!System.Net.Connection.SubmitRequest(System.Net.HttpWebRequest request, bool forcedsubmit) System.dll!System.Net.ServicePoint.SubmitRequest(System.Net.HttpWebRequest request, string connName) System.dll!System.Net.HttpWebRequest.SubmitRequest(System.Net.ServicePoint servicePoint) System.dll!System.Net.HttpWebRequest.GetRequestStream(out System.Net.TransportContext context) System.dll!System.Net.HttpWebRequest.GetRequestStream() RivhitApi.dll!RivhitApi.RivhitService.DoRequest(object objToSend, string Url) Line 59