combobox iteme group by problem ,???

Question

GrpCB.DisplayMember = "Groupe";
           GrpCB.ValueMember = "num_ligne";
           using (DBEntities db = new DBEntities())
           {



               GrpCB.DataSource = db.Grilles.GroupBy(p => p.groupe).ToList();

           }

this my code to add items to cb but i want group by field groupe

 

i have an errror of grouping

 

is there some correction ?