Custom Aspnet Identity and RoleManage

My application is based on using the primary key type int in Aspnet Identity as described here . The problem is when it comes to creating a user. In the original template, I added a property in the RegisterViewModel, associated the Roles in a DropDownList, and it was ready. Now I'm having trouble

Here's a Model:

 public class RegisterViewModel
{

    [Required]
    [Display(Name = "UserRoles")]
    public int UserRoles { get; set; }


    [Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Nome { get; set; }
    ...
}

The Controller

public ActionResult Register()
    {
        ViewBag.Name = new SelectList(context.Roles.ToList(), "Id", "Name");
        return View()
    }

A View Register.cshtml

 <!--Selecione o Tipo de Perfil para o usuário-->
<div class="col-md-6">
    @*@Html.Label("Selecione o Tipo de Perfil do Usuário")
    @Html.DropDownList("UserRoles", (SelectList)ViewBag.Name, "Selecione o perfil de usuário", new { @class = "form-control" })
</div>
<!--termina aqui-->

Roles are displayed in the registry:

IntheRegisterPostthecodeisthis:

if(ModelState.IsValid){varuser=newApplicationUser{UserName=model.Email,Email=model.Email,Nome=model.Nome,Nacionalidade="brasileira", EstadoCivil = model.EstadoCivil, Identidade = model.Identidade, OrgaoExpedidor = model.OrgaoEmissor, CPF = model.CPF, Endereco = model.Endereco, Numero = model.Numero, Complemento = model.Complemento, Bairro = model.Bairro, Cidade = model.Cidade, Estado = model.Estado, DDD_Celular = model.DDD_Celular, Celular = model.Celular, DDD_TelComercial = model.DDD_TelComercial, TelComercial = model.TelComercial, DDD_Telefone = model.DDD_Telefone, DDD_WhatsApp = model.DDD_WhatsApp, PreferenciaContato = model.PreferenciaContato };
            var result = await UserManager.CreateAsync(user, model.Password);
            if (result.Succeeded)
            {
                await this.UserManager.AddToRoleAsync(user.Id, model.UserRoles);
                await SignInManager.SignInAsync(user, isPersistent: false, rememberBrowser: false);
...

Attempting to register gives the following error:

What might be causing this error?