inline RJS ActionControllerException: "stack level too deep"

Hi,

I get a ActionControllerException: "stack level too deep" when I
attempt inline RJS like this:

<%=observe_field("loan_term", :function => update_page { |page|
page[:max_term_in_yrs].insert_html(page[:loan_term].value/12); })%>

What am I missing -- where's the wind?

Thanks,

Lille