Error submitting form with ASP MVC

I'm having a problem submitting a form developed in ASP MVC. Clicking the save button returns the following error message:

System.InvalidOperationException: 'Não existe item ViewData do tipo 'IEnumerable<SelectListItem>' que possui a chave 'DepartamentoSelecionado'.'

What can be this error?

The project is divided into 4 main files. The first is Model

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

namespace projetinho.Models
{
    public class UsuarioModel
    {
        public int Id { get; set; }

        [Required(ErrorMessage = "Um nome deverá ser informado!")]
        [StringLength(maximumLength: 50, MinimumLength = 3, ErrorMessage ="O nome deve conter no mínimo 3 e no máximo 50 caracteres.")]
        public string Nome { get; set; }

        public int Id_Departamento { get; set; }

        public List<SelectListItem> DepartamentoSelecionado { get; set; }

        public UsuarioModel()
        {
        DepartamentoSelecionado = new List<SelectListItem>();
        }   
    }
}

The second file is a class that will manipulate the database

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace projeto.Models
{
public class UsuarioHandler
{
    private SqlConnection con;
    private void Conexao()
    {
        string stringConexao = ConfigurationManager.ConnectionStrings["sqlServer"].ToString();
        con = new SqlConnection(stringConexao);
    }

    public bool CadastraUsuario(UsuarioModel Usulist)
    {
        Conexao();
        string query = "insert into usuario(nome, id_departamento) values ('" + Usulist.Nome + "', '" + Usulist.DepartamentoSelecionado + "')";
        SqlCommand cmd = new SqlCommand(query, con);

        con.Open();
        int i = cmd.ExecuteNonQuery();
        con.Close();

        if (i >= 1)
            return true;
        else
            return false;
    }

//FUNÇÃO QUE LISTA OS DEPARTAMENTOS NO DROPDOWN
        public List<UsuarioModel> DepList()
        {
            Conexao();

            List<UsuarioModel> listaDepartamento = new List<UsuarioModel>();

            string query = "select id, nome from departamento";
            SqlCommand cmd = new SqlCommand(query, con);

            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();

            con.Open();
            adapter.Fill(dt);
            con.Close();

            foreach (DataRow dr in dt.Rows)
            {
                listaDepartamento.Add(new UsuarioModel
                {
                    Id = Convert.ToInt32(dr["id"]),
                    Nome = Convert.ToString(dr["nome"])
                });
            }
            return listaDepartamento;
        }
    }
}

The Controller is defined as follows:

using projeto.Models;
using System.Data;
using System.Web.Mvc;

namespace projeto.Controllers
{
    public class UsuarioController : Controller
    {
        [HttpGet]
        public ActionResult Cadastrar()
        {
            UsuarioHandler usuarioHandler = new UsuarioHandler();
            ViewBag.departamentos = new SelectList(usuarioHandler.DepList(), "id", "nome");

            return View();
        }

        [HttpPost]
        public ActionResult Cadastrar(UsuarioModel Usulist)
        {
            if (ModelState.IsValid)
            {
                UsuarioHandler usuariohandler = new UsuarioHandler();

                if (usuariohandler.CadastraUsuario(Usulist))
                {
                    ViewBag.Msg = "<div class='alert alert-success alert-dismissable'><a href = '#' class='close' data-dismiss='alert' aria-label='close'>×</a> Cadastro realizado com sucesso!</div>";
                    ModelState.Clear();
                }
                else
                {
                    ViewBag.Msg = "<div class='alert alert-danger alert-dismissable'><a href = '#' class='close' data-dismiss='alert' aria-label='close'>×</a> Erro ao cadastrar!</div>";
                }
            }
            return View();
        }
    }
}

And lastly,% w / w%

@model projetinho.Models.UsuarioModel 

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()

    <div class="container">
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            <label>Informe o nome:</label>
            @Html.EditorFor(model => model.Nome, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.Nome, "", new { @class = "text-danger" })
        </div>

        <div class="form-group">
            <label>Selecione um departamento: </label>
             @Html.DropDownListFor(model => model.DepartamentoSelecionado, ViewBag.departamentos as SelectList, new { @class = "form-control" })
        </div>

        <div class="form-group">
            <input type="submit" value="Create" class="btn btn-default" />
        </div>
    </div>
}