I recently encountered this error when I attempted to consume a new hosted WCF service. The service used the Request/Response model and had been properly decorated. The response and request objects were marked as DataContracts and had a specified namespace. My WCF service interface was marked as a ServiceContract and shared the namespace attribute value. Everything should have been fine, right? [ServiceContract(Namespace = "http://schemas.myclient.co... public interface IProductActivationService ......