in `fields_for` when i add `:class=>"street"` to `text_field` it works fine, where as
when i add `:class=>"zip"` to select it is triggring error

```
<%= form_for @person do |person_form| %>
    <%= person_form.fields_for :address do |address_fields| %>
      Street  : <%= address_fields.text_field :street, :class=>"street" %>
      Zip code: <%= address_fields.select :zip_code, :class=>"zip", options_for_select(@zip.to_a) %>
    <% end %>
  <% end %>

```

please, help to solve the issue

thanks