observe(ing)_fild a collection_select with 1 item

Hi there I'm obseving a field (a collection select) and it work if the collection has more than one item, if it has only one it wont change anything here is my code

<%= collection_select 'address', 'address', @addresses, :id, :address %></p> <%= observe_field(               :address_address,               :update => 'shipping',               :url => { :action => :shi },               :complete => "new Effect.SlideDown('shipping')",               :with => 'boo') %>

As I read that obseve_fields take the same arguments of link_to_remote i have tried :on => click focus change but no luck if there is only one item in the collection it wont work

thanks in advance