Error when trying to implement a multiple_select

Hello, I’m trying to make a simple multiple selection in an appointment form, so users don’t have to search the ID for a medic, normally I searched a lot on how to implement a multiple_select and carefully followed everything on many forums, but no one gets this error:

no function clause matching in Phoenix.HTML.Tag.dasherize/1

My controller is as follows:

  defmodule Medica.AppointmentuController do
  use Medica.Web, :controller

  alias Medica.Appointment
  alias Medica.Medico

  def index(conn, _params) do
    appointments = Repo.all(Appointment)
    render(conn, "index.html", appointments: appointments)
  end

  def new(conn, _params) do
    changeset = Appointment.changeset(%Appointment{})
    medico = Repo.all from u in Medico, select: {u.id, u.nombre}
    render(conn, "new.html", medico: medico, changeset: changeset)
  end

  ...

end

New.html.eex

<h2 align="center">Modo Paciente</h2>
<h2>Reservar Nueva Hora</h2>

<%= render "forma.html", changeset: @changeset,
                        action: appointmentu_path(@conn, :create),
                        medico: @medico %>

and form.html.eex:

<%= form_for @changeset, @action,@medico, fn f -> %>
  <%= if @changeset.action do %>
    <div class="alert alert-danger">
      <p>Ingrese los datos correctamente o puede que la cita ya esté tomada.</p>
    </div>
  <% end %>

  <div class="form-group">
    <%= label f, :rut_paciente, class: "control-label" %>
    <%= text_input f, :rut_paciente, class: "form-control" %>
    <%= error_tag f, :rut_paciente %>
  </div>

  <div class="form-group">
    <%= label f, :fecha, class: "control-label" %>
    <%= date_input f, :fecha, class: "form-control" %>
    <%= error_tag f, :fecha %>
  </div>

  <div class="form-group">
    <%= label f, :id_medico, class: "control-label" %>
    <%= multiple_select f, :medico, Enum.map(@medico, &{&1.id, &1.nombre}) %>
    <%= error_tag f, :id_medico %>
  </div>

  <div class="form-group">
    <%= label f, :bloque, class: "control-label" %>
    <%= number_input f, :bloque, min: 1, class: "form-control" %>
    <%= error_tag f, :bloque %>
  </div>

  <div class="form-group">
    <%= label f, :descripcion, class: "control-label" %>
    <%= text_input f, :descripcion, class: "form-control" %>
    <%= error_tag f, :descripcion %>
  </div>

  <div class="form-group" align="center">
    <%= submit "✔", class: "btn btn-primary" %>
    <a class="btn btn-danger" href="/" role="button">✖</a>
  </div>

<% end %>

My models:

  schema "appointments" do
    field :rut_paciente, :string
    field :fecha, :date
    field :id_medico, :integer
    field :bloque, :integer
    field :descripcion, :string

    timestamps()
  end

  schema "medico" do
    field :nombre, :string
    field :apellido, :string
    field :especialidad, :string
  end

Screen for the error:

Screenshot-2018-6-4%20FunctionClauseError%20at%20GET%20appointments_u%20new1346×991 108 KB