Problems with select

Hello to every bofy i'm working with Ruby and i havve the next problem
when i want to select an option from a select.

I have the next code:

    <p>
      <%= f.label :es_pyme, I18n.t('tog_user.model.es_pyme') %>
    <select name="user[es_pyme]" class="fieldbox">
          <option value= "" >
              <%= I18n.t("tog_user.model.select_type_of_user") %>
            </option>
        <option value= "Usuario" > Usuario </option>
        <option value= "Pyme" > Pyme </option>
    </select>
    </p>

and in my model

  validates_presence_of :es_pyme

and when i select an option always appear that es_pyme is null.

Which is the problem?

thanks and greetings.