System.ArgumentException: DbExpressionBinding requiert une expression d'entrée avec le ResultType d'une collection. Nom du paramètre : input

Question

I'm getting this error DbExpressionBinding requires an input expression with a collection ResultType.So I don't understand what is happening and why I am getting this error. Using ADO.Net Entity Model. Entity Framework 6.

            int numCentre =Convert.ToInt32(GridView1.Rows[e.NewSelectedIndex].Cells[1].Text);
        var resultat = from ca in dc.Centre_Artisanal
                       join at in dc.Ateliers on ca.num_centre_artisanal equals at.num_centre_artisanal
                       join la in dc.Location_Atelier on at.idatelier equals la.idatelier
                       join ar in dc.Artisans on la.cin equals ar.cin
                       where ca.num_centre_artisanal==numCentre
                       group new { ca, at, la, ar } by new
                       {
                           ca.num_centre_artisanal,
                           ca.nom_centre_artisanal,
                           ca.nom_porteur_projet,
                           ar.cin,
                           ar.situation_familiale
                       } into gr
                       let veuves = gr.Where(g => g.ar.situation_familiale == "V")
                       select new
                       {
                           num=gr.Key.num_centre_artisanal,
                           nom=gr.Key.nom_centre_artisanal,
                           porteur=gr.Key.nom_porteur_projet,
                           nbArtisans = gr.Key.cin.Count(),
                           nbveuves = veuves.Count()
                       };
        GridView2.DataSource = resultat.ToList();
        GridView2.DataBind();