asp.net MVC4 checkbox

View

@Html.Bootstrap().FormGroup().CheckBoxFor(t => t.Repetir).IsChecked(false).HtmlAttributes(new { @Value = true })

Controller

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(DespesaViewModel despesaViewModel)        
    {
        if (ModelState.IsValid)
        {

            DespesaModels despesaModels = AutoMapper.Mapper.Map<DespesaViewModel, DespesaModels>(despesaViewModel);
            db.DespesaModels.Add(despesaModels);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        return View(despesaViewModel);
    }

ViewModel

    [Required]
    public bool Repetir { get; set; }

It's ok to save a new register, but if(ModelState.IsValid) return false --(other fields validation failed)-- when page postback to load validation error messages i get error in my checkBoxFor line (in view):

An exception of type 'System.NullReferenceException' occurred in MyProject.Bootstrap.dll but was not handled in user code

It's seems that my bool property Repetir can't pass (bool)True value to checkBox, but i'm not shure what is going on.