Error executing JSON based Restful WCF Service - json

Question

I am trying to write a json based restful WCF service. When i execute it using RestSharp i get a null response with the following Error in the ErrorException and the ErrorMessage.
Data at the root level is invalid. Line 1, position 1.
Status Code is BadRequest
The data object is created as
[DataContract]
public class User
{
[DataMember]
public string first_name { get; set; }
[DataMember]
public string last_name { get; set; }
[DataMember]
public string email_address { get; set; }
[DataMember]
public List<UserAddress> Address; //do not serialize this in the data contract
}
[DataContract]
public class UserAddress
{
[DataMember]
public string line_1_address { get; set; }
[DataMember]
public string state { get; set; }
[DataMember]
public string PostalArea { get; set; }
}
The service code is
[ServiceContract]
public interface IUserService
{
[OperationContract]
User AddUser(User user);
}
public class UserService : IUserService
{
[WebInvoke(Method = "PUT",
RequestFormat = WebMessageFormat.Json,
ResponseFormat = WebMessageFormat.Json,
UriTemplate = "User/")]
public User AddUser(User user)
{
return new User();
}
}
The service is hosted in a Web App.
I have a c# client based on RestSharp
static void Main(string[] args)
{
User u = new User();
u.first_name = "testFirst";
u.last_name = "testLast";
UserAddress ua = new UserAddress();
ua.line_1_address = "line 1";
ua.PostalArea = "08648";
ua.state = "NJ";
u.Address = new List<UserAddress>();
u.Address.Add(ua);
RestSharp.RestClient client = new RestSharp.RestClient("http://localhost/TestWCHHost/User.svc");
var request = new RestRequest(String.Format("User/"), Method.PUT);
request.RequestFormat = DataFormat.Json;
request.AddObject(u);
var response = client.Execute<User>(request);
}
The web config changes are as
<?xml version="1.0"?>
<configuration>
<appSettings>
<add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
</appSettings>
<system.web>
<compilation debug="true" targetFramework="4.5" />
<httpRuntime targetFramework="4.5"/>
</system.web>
<system.serviceModel>
<services>
<!-- User Service -->
<service name="TestService.UserService" behaviorConfiguration="serviceBehavior">
<endpoint address="" binding="webHttpBinding" contract="TestService.IUserService" behaviorConfiguration="web">
</endpoint>
</service>
</services>
<behaviors>
<serviceBehaviors>
<behavior name="serviceBehavior">
<serviceMetadata httpGetEnabled="true"/>
<serviceDebug includeExceptionDetailInFaults="true"/>
</behavior>
</serviceBehaviors>
<endpointBehaviors>
<behavior name="web">
<webHttp/>
</behavior>
</endpointBehaviors>
</behaviors>
</system.serviceModel>
<system.webServer>
<modules runAllManagedModulesForAllRequests="true"/>
<!--
To browse web app root directory during debugging, set the value below to true.
Set to false before deployment to avoid disclosing web app folder information.
-->
<directoryBrowse enabled="true"/>
</system.webServer>
</configuration>
Unable to figure whats going on. Any clues.