option :prompt=>true from f.select not working on edit action

I'm having a bit of a trouble on my edit form. For some reason my select helper is not working properly on the edit action. It seems to ignore for some reason the option :prompt=>true and does not show the "Please select" message. But only on the edit action, the new action is working just fine.

the snippet from the form partial:

f.select :category_id, @categories, {:prompt => true}

the edit action

  def edit
    @page = Page.find(params[:id])
    @categories = Category.where(:cat_type=>"page").map { |c| [t("category.#{c.slug}",:default=>"#{c.name}"), c.id] }
  end

the new action

  def new
    @page = Page.new
    @categories = Category.where(:cat_type=>"page").map { |c| [t("category.#{c.slug}",:default=>"#{c.name}"), c.id] }

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @page }
    end
  end

Any idea?