Invalid Party Object Type 4002

I am trying to create a Service Appointment though code.  I am developing against Dynamics 365.

I am using a resource (as apposed to a system user in the Resource Party List.

For some reason I am getting the following error:

"An exception of type 'System.ServiceModel.FaultException`1' occurred in Microsoft.Xrm.Sdk.dll but was not handled in user code

Additional information: Invalid Party Object Type 4002"

Yet the Service Activity should be able to accept the resource. 

Here is my code.  Hopefully it provides enough information for someone to see what I have done wrong here:

public Guid CreateAndReserveAppointment(AppointmentProposal proposal, Guid resourceId, Guid serviceId, Guid incidentId)
        {
            Guard.ArgumentIsNotNull(proposal, "proposal");

            ActivityParty resourceAP = new ActivityParty
            {
                PartyId = new EntityReference(Resource.EntityLogicalName, resourceId)
            };

            ServiceAppointment appt = new ServiceAppointment()
            {
                Subject = RESERVED_APPT_SUBJECT,
                ScheduledStart = proposal.Start,
                ScheduledEnd = proposal.End,
                ServiceId = new EntityReference(xrmHelper.Service.EntityLogicalName, serviceId),
                RegardingObjectId = new EntityReference(Incident.EntityLogicalName, incidentId),
                Resources = new ActivityParty[] { resourceAP }
            };

            CreateRequest req = new CreateRequest
            {
                Target = appt
            };

            CreateResponse resp = (CreateResponse)Service.Execute(req);

            if (resp.id.Equals(Guid.Empty))
                throw new ApplicationException("The system was unable to reserve the Appointment.");

            ReserveAppointment(resp.id);
            return resp.id;
        }