Encounter Error for generating Radio button from Model

Encounter Error for generating Radio button from Model

i am trying to generate radio button from model but getting object
reference error. i just can not understand where i am making mistake in
MVC because i am just learning it. here is my full code. please have a
look and tell me where is the mistake. thanks
model class
public class StudentModel
{
[Required(ErrorMessage = "First Name Required")] // textboxes will
show
[Display(Name = "First Name :")]
[StringLength(5, ErrorMessage = "First Name cannot be longer than
5 characters.")]
public string FirstName { get; set; }
[Required(ErrorMessage = "Last Name Required")] // textboxes will
show
[Display(Name = "Last Name :")]
[StringLength(5, ErrorMessage = "Last Name cannot be longer than 5
characters.")]
public string LastName { get; set; }
[Required(ErrorMessage = "Sex Required")]
[Display(Name = "Sex :")]
public int SexID { get; set; }
public List<Sex> Sex { get; set; }
}
public class Sex
{
public string ID { get; set; }
public string Type { get; set; }
}
controller class
public class HomeController : Controller
{
[HttpGet]
public ActionResult Index()
{
ViewBag.Message = "Welcome to ASP.NET MVC!";
var student = new StudentModel
{
FirstName = "Rion",
LastName = "Gomes",
//I think the best way to populate this list is to call a
service here.
Sex = new List<Sex>
{
new Sex{ID="1" , Type = "Male"},
new Sex{ID="2" , Type = "Female"}
}
};
return View();
}
[HttpPost]
public ActionResult Index(StudentModel model)
{
if (ModelState.IsValid)
{
//TODO: Save your model and redirect
}
//Call the same service to initialize your model again (cause
we didn't post the list of sexs)
return View(model);
}
}
view code
@model MvcRadioButton.Models.StudentModel
@Html.BeginForm()
{
<div>
@Html.LabelFor(model => model.FirstName)
@Html.EditorFor(model => model.FirstName)
@Html.ValidationMessageFor(model => model.FirstName)
</div>
<div>
@Html.LabelFor(model => model.LastName)
@Html.EditorFor(model => model.LastName)
@Html.ValidationMessageFor(model => model.LastName)
</div>
@{
foreach (var sex in Model.Sex)
{
<div>
@Html.RadioButtonFor(model => model.Sex, new { id = "sex"
+ sex.ID })
@Html.Label("sex" + sex.ID, sex.Type)
</div>
}
}
<input type="submit" value="Submit" />
}
this line throwing error
foreach (var sex in Model.Sex) saying model null or object reference error