I tried :include_blank => true, but it didn\'t work.
:include_blank => true
<%= options_for_select Model.all.collect{|mt| [mt.name, mt.id]} %&
= select_tag "some_value", options_for_select(Model.all.collect{ |x| [x.name, x.id]}.prepend([t('helpers.some_name'), nil]), :selected => params[:some_value])