DropDownListFor - why selected element is always the first?

I have a DropDownListFor in my View:

@model UserProfileViewModel

@Html.DropDownListFor(x => x.PreferredLangID,
    new SelectList(Model.PreferredLanguages,
        "LanguageID",
        "DisplayName",
        Model.PreferredLang))

Controller action:

 public ActionResult EditProfile()
 {
     List<Language> languages = Code.LanguageLogic.GetAllLanguages();
     UserProfile profile = Code.ProfileLogic.GetUserProfile();
     var viewModel = new UserProfileViewModel
     {
         UserID = profile.UserID,
         PreferredLanguages = languages,
         PreferredLang = profile.PreferredLang
     };
     return View(viewModel);
 }

I thought, that if I gave Model.PreferredLang as 4th paramater in SelectList's contructor, that element will be selected in dropdown, when page loads. But selected element is always the first one in list.

The Model:

public class UserProfileViewModel
{
    public Guid UserID { get; set; }
    public Language PreferredLang { get; set; }
    public int PreferredLangID { get; set; }
    public IEnumerable<Language> PreferredLanguages { get; set; }
    //...
}