The parameter conversion from type 'System.String' to type 'RamalAguia.Models.Setor' failed because no type can convert between these types

While performing a project I came across the following error:

  System.InvalidOperationException: The parameter conversion from type 'System.String' to type 'RamalAguia.Models.Setor' failed because no type can convert between these types.

This is my Controller:

    [HttpGet]
    public ActionResult Create()
    {
        PreparaFormulario();            
        return View();
    }
    [HttpPost]
    public ActionResult Create(RamalModel model)
    {
        if (ModelState.IsValid)
        {
            _db.Ramais.Add(model);
            _db.SaveChanges();
            return RedirectToAction("Index", new { id = model.ID });
        }
        else
        {
            PreparaFormulario();
            return View();
        }

    }    

    [HttpGet]
    public void PreparaFormulario()
    {
        var setores = new List<Setor>();
        using (RamaDb db = new RamaDb())
        {
            setores = db.Setores.ToList();
        }
        ViewBag.ID2 = new SelectList(_db.Setores, "setorID", "setorNome");


    }

A View:

    @Html.DropDownListFor(model => model.setores, (SelectList)ViewBag.ID2)

And the Model:

 using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Linq;
 using System.Web;

 namespace RamalAguia.Models
 {
 public class RamalModel
 {
    public int ID { get; set; }

    [Required(ErrorMessage = "O nome é obrigatório.")]
    public string Nome { get; set; }

    [Required(ErrorMessage = "O número é obrigatório.")]
    public int Numero { get; set; }



    public Setor setores { get; set; }

How can I fix it?

Edit

Obs: I was able to find this error through a breakpoint in ModelState.IsValid, when the application will save the data passed, within the values I use to save the sector. >

Industry Model

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;

 namespace RamalAguia.Models
 {
    public class Setor
    {
       public int setorID { get; set; }

       public string setorNome { get; set; }

       public Area areas { get; set; }
    }
}

Edit2:

Classes:

RamalModel

    public class RamalModel
    {
    public int ID { get; set; }

    [Required(ErrorMessage = "O nome é obrigatório.")]
    public string Nome { get; set; }

    [Required(ErrorMessage = "O número é obrigatório.")]
    public int Numero { get; set; }


    public int SetorId { get; set; }

    [ForeignKey("SetorId")]
    public Setor setores { get; set; }

Sectors

    public class Setor
    {
    public int setorID { get; set; }

    public string setorNome { get; set; }

    public Area areas { get; set; }

And dbcontext

    namespace RamalAguia.Models
    {
        public class RamaDb : DbContext
        {
             public DbSet<Usuario> Usuarios { get; set; }

             public DbSet<RamalModel> Ramais { get; set; }

             public DbSet<Setor> Setores { get; set; }

             public DbSet<Area> Areas { get; set; }
        }
    }