ModelState.Isvalid is always false

my model:

public class Unidad
{
    public int UnidadID { get; set; }

    [Required(ErrorMessage ="Unidad Abrevidad no puede ser vacio")]
    [StringLength(50)]
    [Display(Name ="Unidad Abrevidad")]
    public string AbbreviatedUnit { get; set; }

    [Required(ErrorMessage ="Descripcion no puede ser vacio")]
    [StringLength(200,MinimumLength =3)]
    [Display(Name = "Descripción")]
    public string description { get; set; }

}

located in the controller my method delete

public ActionResult Delete(int id, Unidad unidad)
{
     try
        {
            if (ModelState.IsValid)
            {
                Unidad deleteunidad = TempRepository.EditOrDelete(id);
                if (deleteunidad == null)
                {
                    return HttpNotFound();
                }
                TempRepository.DeleteUnidad(deleteunidad);
                return RedirectToAction("Index");
            }

            // TODO: Add delete logic here

           return View(unidad);
        }
        catch
        {
            return View(unidad);
        }
    }

located in the UnitRepository class my method to find unity

internal Unidad FindUnidad(int? id)
    {
        using (var db = new CNRContext())
        {
            var unidad = db.Unit.Find(id);
            return unidad;
        }
    }

my method to erase the unit

internal void DeleteUnidad(Unidad unidad)
    {
        using (var db = new CNRContext())
        {

            db.Entry(unidad).State = EntityState.Deleted;
            db.SaveChanges();
        }
    }

my ModelState.IsValid is always false when I try to delete a unit ??