rails check_box_tag disapear

I want to make a form based on check_boxes, and I want it to be checked, if '@clans' array is not nil and it contains a 'value' of a given checkbox.

My first atemnpt was:

<%= check_box_tag 'clans[]','Feniks', true if !@clans.nil? and @clans.include? 'Feniks' %>

but when @clans is nil, the check_box disappears, just like it was disabled.

I managed to solve it with code like that:

  <% if !@clans.nil? and @clans.include? 'Feniks' %>
    Feniks: <%= check_box_tag 'clans[]','Feniks', true %>
  <% else %>
    Feniks: <%= check_box_tag 'clans[]','Feniks' %>
  <% end %>

but it looks so bad, that there have to be a better way to write this :)

Please show me how to handle this :)