Rails 4 : Checkbox array is not updating my attribute

My starting place was this discussion: Syntax for form_for when building an array from checkboxes

I have a call to my model passing back an array of valid options. This array then makes a series of check_box_tag

<%= form_for @game, :url => wizard_path do |f| %>

  <div>
  <% @game.select_races.each do |a| %>
    <%= f.label a %>
    <%= check_box_tag 'game[races][]', a , true %>
  <% end %>
  </div>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

This successfully creates an array called 'races' containing the desired output. The problem is that it doesn't actually update the races attribute. So my races attribute is still nil.

I'm sure this is a painful Rails beginner question. Any help is appreciated.

UPDATE

My allowed params were:

def game_params
  params.require(:game).permit(:shattered_empire, :shards_of_the_throne, :number_of_players, :rules, :strategy_cards, :players, :races)
end

Which needed to be updated to:

def game_params
  params.require(:game).permit(:shattered_empire, :shards_of_the_throne, :number_of_players, :rules, :strategy_cards, :players, {:races => []})
end