CRM 2011 error message Request not supported

I am getting an error message of “Request not supported” when trying to make a soap request in Silverlight application. Following is the code I am using

        EntityReference entityReference = new Xrm.Sdk.EntityReference("userquery",new Guid("{000000-0000-0000-0000-000000000000}"));

           
            ParameterCollection parameterCollection = new ParameterCollection();

            parameterCollection.Add("Target", entityReference);

            OrganizationRequest req = new OrganizationRequest

            {

                RequestName = "RetrieveSharedPrincipalsAndAccessRequest",

                Parameters = parameterCollection

            };

            OrganizationResponse response = _serviceProxy.Execute(req);

Following code works fine if I use early bound types

            

           EntityReference entityReference = new Xrm.Sdk.EntityReference("userquery",new Guid("{00000000-0000-0000-0000-000000000000}"));

            RetrieveSharedPrincipalsAndAccessRequest req = new RetrieveSharedPrincipalsAndAccessRequest();

            req.Target = entityReference;

            OrganizationResponse response = _serviceProxy.Execute(req);

As I am making a soap calls in silverlight utility, later option is not applicable and I am only left with first option to use. 

Solution: I was using the RequestName wrong there should not be any "Request" name appended at the end of it i.e. use RetrieveSharedPrincipalsAndAccess instead of RetrieveSharedPrincipalsAndAccessRequest