Error in Dropdownlist validation

        View

      @Html.LabelFor(M => Model.Patient.DoctorID)
      @Html.DropDownList("doctorType")


             Controller

      public List<SelectListItem> SelectDoctor()
{
    Gema_Doctor[] doctorList;
    doctorList = gema_Doctor.GetDoctor().ToArray();
    List<gema_doctor> listDoctor = new List<gema_doctor>(doctorList);
    List<SelectListItem> dropItems = new List<SelectListItem>();
    dropItems.Add(new SelectListItem { Text = "--Select--", Value = "", Selected = true });
    foreach (Gema_Doctor doctorValues in listDoctor)
    {
        dropItems.Add(new SelectListItem { Text = doctorValues.Dr_Name, Value = doctorValues.Dr_Id.ToString() });
    }
    return dropItems;
}  .

              Model
     [Required(ErrorMessage="Please Select Doctor")]
     [Display(Name = "DoctorID")]
     public int DoctorID { get; set; }