QuickBlox create user issue (bad request) without any error message - quickblox

Question

I want to create an user using the rest API. Rest API returns an error (bad request), when I made the request. There are not any other error messages.
JSON:
Required Parameters : login, password, email
http://quickblox.com/developers/Users#API_User_Sign_Up
{
"user":
{
"login":"user123456",
"password":"User11#2015",
"email":"xxx#ccc.com.tr",
"blob_id":null,
"external_user_id":null,
"facebook_id":null,
"twitter_id":null,
"full_name":null,
"phone":null,
"website":null,
"custom_data":null,
"tag_list":null
}
}
Rest API Result:
Response Status Code : BadRequest.
Response Content is empty. No error message.
I do not understand the error. Can you help me?
Code :
public NotificationUser CreateUser(string token, NotificationUser notificationUser)
{
var user = new QuickbloxUser()
{
email = notificationUser.Email,
password = notificationUser.Password,
login = notificationUser.Login
};
var jsonBody = JsonConvert.SerializeObject(user);
var request = new RestRequest("users.json", Method.POST);
request.AddHeader("QB-Token", token);
request.AddParameter("user", jsonBody);
var response = _restClient.Execute<RootUser<QuickbloxUserResponse>>(request);
if (response.StatusCode == HttpStatusCode.OK)
{
notificationUser.Id = response.Data.user.id;
notificationUser.Email = response.Data.user.email;
notificationUser.Login = response.Data.user.login;
return notificationUser;
}
return null;
}