Cannot link a Contact to Campaign Response in CRM 2011

Hi,

Anyone know in CRM 2011 how to link a Contact to Campaign Response customer field programmatically? I have tried below method but no luck

      ActivityParty activityParty = new ActivityParty()                {                    PartyId = new EntityReference("contact", guidContactId),                     ActivityId = new EntityReference                    {                        Id = Guid.NewGuid(),                        LogicalName = ActivityPointer.EntityLogicalName,                    },                    ParticipationTypeMask = new OptionSetValue(11)                };

 campaignresponse["customer"] = new ActivityParty[] { activityParty };

it keep giving me error : "Value cannot be blank \r\n Parameter : value"