Trouble with Using Roles

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C#

ASP.NET

MVC

This is my code of login(if it's Admin he will give him Admin Role) :

[HttpPost]
       public ActionResult Logpage(LogModel.Login Model)
       {
           if (ModelState.IsValid)
           {

               if (LogModel.Login.Verifuser(Model.IDUser, Model.Password))
               {
                   if (LogModel.Login.IsActive(Model.IDUser, Model.Password))
                   {
                       FormsAuthentication.SetAuthCookie(Model.IDUser, false);
                       if (LogModel.Login.IsAdmin(Model.IDUser, Model.Password))
                       {
                           Roles.AddUserToRole(Model.IDUser, "Admin");
                           return RedirectToAction("Index", "Admin");
                       }
                       else
                       {
                           return RedirectToAction("Index", "Agence",Model.IDUser);
                       }
                   }
                   else
                   {
                       ModelState.AddModelError("", "Compte Desactive Vieullez contactez l'admin SVP");
                   }
               }
               else
               {
                   ModelState.AddModelError("", "Invalide username or Password");
                 return View(Model);
               }
           }
           return View(Model);
       }

the problem that on this line Roles.AddUserToRole(Model.IDUser, "Admin"); i got this error System.Configuration.Provider.ProviderException: .