Modelstate invalida

Question

I have a class called, Colaborador. and the collaborator has a bank.

 public class Colaborador
 {
  public virtual Banco Banco { get; set; }
  public Guid? BancoId { get; set; }
 }

my problem, is that even though the bank is optional, invalidates the modelState, because some fields of the Bank class, are mandatory.

I tried to do so:

if (colaboradorViewModel.Banco.NomeDoBanco == null)
            {
                colaboradorViewModel.Banco = null;
            }

            if (!ModelState.IsValid)
            {
                ViewBag.Estado = new SelectList(_estadoAppService.ObterTodos(), "EstadoId", "Uf");
                ViewBag.Usuarios = new SelectList(_usuarioAppService.ObterTodos(), "UsuarioId", "Nome");
                return View(colaboradorViewModel);
            }

even so, it keeps validating the attributes of the object banco. how do I fix it??

My Collaboratordoconfi, it’s like this::

public class ColaboradorConfig: EntityTypeConfiguration<Colaborador>
    {
        public ColaboradorConfig()
        {
            ToTable("Colaboradores");
            HasKey(c => c.ColaboradorId);

            HasOptional(c => c.Banco).WithMany().HasForeignKey(b => b.BancoId);

        }
    }

Banco Config:

   public class BancoConfig: EntityTypeConfiguration<Banco>
    {
        public BancoConfig()
        {
            ToTable("Bancos");
            HasKey(b => b.IdBanco);
            Property(b => b.NomeDoBanco).HasMaxLength(50).IsRequired();
            Property(b => b.Agencia).HasMaxLength(10).IsRequired();
            Property(b => b.Conta).HasMaxLength(10).IsRequired();
            Property(b => b.DigitoAgencia).IsOptional();
            Property(b => b.Digito).IsOptional();
            Property(b => b.Op).IsOptional();
        }
    }