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(
              :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
but no luck
if there is only one item in the collection it wont work

thanks in advance