namespace WcfRestService2.Model  
{  
    [DataContract]
    public class InternalInfoEW : ServiceInfoEW
    {   
        [DataMember]
        public bool? Good { get; set; }

        [DataMember]
        public int Loop { get; set; }

        [DataMember]
        public Dealer DealerInf { get; set; }

        [DataMember]
        public IList<CommentsHistory> Commenter { get; set; }

        [DataMember]
        public IList<CommentsHistory> Comments { get; set; }

        [DataMember]
        public IList<CommentsHistory> CommentsInternal { get; set; }

        [DataMember]
        public IList<ReplyInfoHistory> SRReplyHistory { get; set; }
    }

}

新建(Http/POST)，POST里的数据格式通过RequestFormat定义为Json，WCF框架接受到Json数据的请求，会自动反序列化成InternalInfoEW实例。但是反序列化失败，报错object reference not set to an object,不知道为什么？求高手指点
代码如下：
[WebInvoke(UriTemplate = "Tasks/Add", Method = "POST",   
    RequestFormat=WebMessageFormat.Json)]  
public void Create(InternalInfoEW pocoTask)  
{  
    var ctx = WebOperationContext.Current;  

    try  
    {  
        using (var db = new TasksEntities())  
        {  

        }  

    }  
    catch (Exception ex)  
    {  
        ctx.OutgoingResponse.StatusCode = System.Net.HttpStatusCode.ExpectationFailed;  
        ctx.OutgoingResponse.StatusDescription = ex.Message;  
    }  
}