could not be loaded because it has a parameter or return type of type System.ServiceModel.Channels.Message or a type that

Question

All, i'm getting following error while loading WCF Sevice. The operation 'GetEffort' could not be loaded because it has a parameter or return type of type System.ServiceModel.Channels.Message or a type that has MessageContractAttribute and other parameters of different types. When using System.ServiceModel.Channels.Message or types with MessageContractAttribute, the method must not use any other types of parameters.

Here is service contract

 [OperationContract]
 [WebInvoke(Method = "GET", ResponseFormat = WebMessageFormat.Json,
 BodyStyle = WebMessageBodyStyle.Bare, UriTemplate = "/GetEffort?date={date}&empoyeeID={empoyeeID}")]
 Message GetEffort(DateTime date, int empoyeeID);

i have no clue what is going on. I see similar posts which says message contract mismatch. But here I'm returning a type Message itself.