DLL hatası

Aşağıdaki kod "The Given Key is not present in dictionary" hatası veriyor. Nasıl çözebilirim.

public static Dictionary<string, object> SMS_GONDER(List<Dictionary<string, object>> parameters)
{
    var result = new Dictionary<string, object>();

    var List = new List<Dictionary<string, object>>();

    result["Error"] = "";

    try

    {
     string phoneNumbers = (string)parameters[0]["telefonno"];
        string message = (string)parameters[1]["mesaj"];
       
        
        //var url = "http://10.33.2.33:10080/api/v1/sms/send";
        var url = "https://webhook.site/08ee247b-f938-4cd8-b8d3-e8737bf8058e";
       
        using (var wb = new WebClient())
        {
            var data = new NameValueCollection();
            data["phoneNumbers"] = phoneNumbers.ToString();
            data["message"] = message.ToString();
            var response = wb.UploadValues(url, "POST", data);
            string responseInString = Encoding.UTF8.GetString(response);
            List.Add(new Dictionary<string, object> { { "result", responseInString } });
        }
       
        result["List"] = List;

    }

    catch (Exception ex)

    {

        result["Error"] = ex.Message;

    }

    return result;
   
    
}