Validation for dropdownlist doesn't work in mvc

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MVC4

I have created one form for validation using data annotations.If no delection made in dropdownlist ;it should validate form.
controller:

public ActionResult Create()
{

    ViewBag.GenreId = new SelectList(objdata.Genres, "GenreId","Name");
    ViewBag.ArtistId = new SelectList(objdata.Artists, "ArtistId", "Name");
    return View();
}
[HttpPost]
public ActionResult Create(Album album)
{
    if (ModelState.IsValid)
    {
        objdata.Albums.Add(album);
        ///  Another way
        // objdata.Entry(album).State = EntityState.Added;
        objdata.SaveChanges();
        return RedirectToAction("Index");
       // return View(album);
    }
   return View(album);


}

model

public class Album
  {
            [DisplayName("Genre")]
           [Required(ErrorMessage = "pls select Genre")]
            public int GenreId { get; set; }
           [DisplayName("Artist")]
            [Required(ErrorMessage = "pls select Artist")]
            public int ArtistId { get; set; }

      }

View


@using (Html.BeginForm())
{
@Html.ValidationSummary(true)
{

Album




}
}


validation doesn't work if i don't select anything from dropdwnlist