mvc 4 Model is null

mvc 4 Model is null

i'm using razor's listboxfor for the first time, but my Model is always
null. after reading similar posts and tryouts it still won't work.
Person.cshtml
@model MVCApplication.Web.ViewModel.PersonViewModel <p>
@using (Html.BeginForm("Index", "Person", FormMethod.Get, new { enctype =
"multipart/form-data" }))
{
//Model.ListboxData -> Model is null
@Html.ListBoxFor(model => model.ListboxData, Model.ListboxData);
} </p>
PersonController.cs
[HttpGet]
[ValidateAntiForgeryToken]
public ActionResult Index()
{
var data = new List<PersonViewModel>();
data.Add(new PersonViewModel{PersonId = 123});
var viewModel = new PersonViewModel()
{
ListboxData = data.AsEnumerable().Select(s => new
SelectListItem{Value=s.PersonId.ToString(),Text =
s.PersonId.ToString()}),
};
return View(viewModel);
}
PersonViewModel
public int PersonId{ get; set; }
private IEnumerable<SelectListItem> listboxData;
public IEnumerable<SelectListItem> ListboxData
{
get
{
return this.listboxData;
}
set
{
this.listboxData = value;
}
}
Thanks a lot for your help!