filling list, but getting Object refrence error for bind dropdownlist in mvc

Question

I am using below code to bind the dropdownlist, i am not getting why i am getting object reference error, even i initialized the entity.

public class RankingEntity
{
    public string Tournament { get; set; }
    public IList<SelectListItem> Years { get; set; }
}

public class Years
{
    public int Year { get; set; }
    public string YearDisplay { get; set; }
}

Controller's code:

 public ActionResult Index()
    {

        var years = _rankingService.GetAllYears().ToList();

        var model = new RankingEntity();
        if (years != null)
        {
            foreach (var y in years)
            {
                model.Years.Add(new SelectListItem
                {
                    Text = y.Year.ToString(),
                    Value = y.Year.ToString()
                });
            }
        }
        return View(model);
    }

I am getting error on model.Years.Add(new SelectListItem i.e. "Object reference"

please correct me,