In Silverlight project I have this exception when I tried to Add a new object to a DataGrid when a button is clicked. In the DomainService class.. I know I have to implement the Add operation for the new Entity I'm putting, but how can I do that? I mean I did the class, the get method but how do I do the insert operation, I can't see my class in this.ObjectContext, so to whom I would be adding this new object, I have the next fragments of code:

public partial class SisPer
    {
        [Key]
        public int Id { get; set; }
        public string Nombre_Sistema { get; set; }
        public string Nombre_Perfil { get; set; }
        public string Nivel { get; set; }
        public bool Estatus { get; set; }
    }

 public IQueryable<SisPer> Get_SisPer()
        {

            var query =
                   from per in this.ObjectContext.Cat_Perfil
                   join sis in this.ObjectContext.Cat_Sistema
                   on per.Cat_Sistema.Id equals sis.Id

                   select new SisPer()
                   {
                       Id = per.Id,
                       Nombre_Sistema = sis.Nombre,
                       Nombre_Perfil = per.Nombre,
                       Nivel = per.Nivel,
                       Estatus = per.Estatus
                   };

        return query;
    }
public void InsertSisPer(SisPer sisper)
        {
          ?? I can't see this.ObjectContext.AddToSisPer();
        }

Plz Help!!