Restsharp Doesn't Deserialize Enumeration

Question

I'm trying to call an external RESTful API in my application and I've been using restsharp for it which has been great so far so but I think I've ran into a problem.

I'm calling an API which normally returns XML documents like this:

<res>
    <resultCode>100</resultCode>
    <resultText>OK</resultText>
    <messageId>52788198</messageId>
    <sessionId>TEST</sessionId>
    <operatorCode>22802</operatorCode>
</res>

This is my code:

internal class SubmitMessageResponse
{
    public ResultCode resultCode { get; set; }
    public string resultText { get; set; }
    public string messageId; { get; set; }
    public string sessionId; { get; set; }
    public string operatorCode; { get; set; }
}

public SubmitMessageResponse SendWelcomeSMS(string subscriptionId, string msisdn)
{
    var request = new RestRequest(Method.GET);
    request.AddQueryParameter("command", "submitMessage")
            .AddQueryParameter("username", _Config.User)
            .AddQueryParameter("password", _Config.Password)
            .AddQueryParameter("msisdn", "00" + _Club.CountryCode + msisdn)
            .AddQueryParameter("businessNumber", _Club.SubscribeShortCode)
            .AddQueryParameter("content", _Club.WelcomeMessage)
            .AddQueryParameter("price", "0")
            .AddQueryParameter("sessionId", subscriptionId)
            .AddQueryParameter("label", "W")
            .AddQueryParameter("keyword", _Club.SubscribeKeyword)
            .AddQueryParameter("operatorCode", "test");

    return SendGetRequest<SubmitMessageResponse>(request);
}

private T SendGetRequest<T>(RestRequest request) where T : new() 
{
    try
    {
        var client = new RestClient(this._BaseUrl);
        client.Timeout = 10000;
        _Logger.DebugFormat("{0} Request: {1}", "submitMessage", client.BuildUri(request));
        var data = client.Execute<T>(request);
        _Logger.DebugFormat("{0} Response: {1}", "submitMessage", Newtonsoft.Json.JsonConvert.SerializeObject(data.Content));
        return data.Data;
    } catch(Exception e)
    {
        _Logger.Error(e);
    }
    return default(T);
}

Where ResultCode is an enumeration including all known values that this API returns:

internal enum ResultCode
{
    Ok = 100,
    AuthorizationInProgress = 150,
    PaymentAuthInProcess = 150,
    InvalidOperatorCode = 201,
...
}

The problem is that normally this would work correctly but I'm unable to deserialize the enum from the XML:

var response = apiHelper.SendWelcomeSMS(client.ExternalData, client.Number);            
Logger.Debug(Newtonsoft.Json.JsonConvert.SerializeObject(response));

My log:

{"messageId":"52788292","sessionId":"TEST","operatorCode":"22802","**resultCode**":0,"resultText":"OK"}

But I also log the response and it works fine:

submitMessage Response: "<res>\r\n\t<**resultCode**>**100**</**resultCode**>\r\n\t<resultText>OK</resultText>\r\n\t<messageId>52788292</messageId>\r\n\t<sessionId>TEST</sessionId>\r\n\t<operatorCode>22802</operatorCode>\r\n</res>\r\n"

Any advice is appreciated.