observe_field producing nothing on view

rubyonrails-talk
1

I had this working fine before, but now for some reason

<%= observe_field( :sale_sku_id, :url => {:action => "update_sales_form"}, :update => :unit_price, :with => :sku_id) %>

Produces nothing on the view side.

I have

<%= javascript_include_tag :defaults %>

at the top of my layouts/application.rhtml

I even checked the code and prototype.js is included:

<script src="/javascripts/prototype.js?1222459662" type="text/javascript"></script>

any ideas?