Restsharp Exception无法识别URI前缀

我从RestSharp The URI prefix is not recognized得到一个异常，不知道为什么。

参数

requestBody只是一个POCO

domain为some.company.com，

_config.AddPersonEndpoint是api/person/add

public PersonResponse AddPerson(PersonRequest requestBody, string domain)
{
    var client = new RestClient(domain);
    var request = new RestRequest(_config.AddPersonEndpoint, Method.POST);
    request.RequestFormat = DataFormat.Json;
    request.AddBody(requestBody);
    var response = client.Execute<PersonResponse>(request);
    if (response.ErrorException != null)
        throw response.ErrorException;
    if (response.ErrorMessage != null)
        throw new PortalConnectivityException(response.ErrorMessage);
    if (!string.IsNullOrEmpty(response.Data.ErrorMessage))
        throw new PortalException(response.Data.ErrorMessage);
    return response.Data;
}