Problem with in_place_checkbox

I need to use in_place_checkbox. I have tried the inplacecontrols
plugin, but it is not working. Here is my code (some of it is dutch, in
case you’re wondering):

Verwerken rekeningen

<%= start_form_tag({:action => "get_invoices"}, :id => "entry-form") %>

Maand
<%= select_year(Date.today, :start_year => Schoolyear.get_startdate.year, :end_year => Schoolyear.get_enddate.year)%> <%= select_month(Date.today)%>

<%= Parent.human_attribute_name("payment")%>
<%= select(:parent, :payment, %w{ 6 2 1}) %>

<%= end_form_tag %>

<%= observe_form “entry-form”,
:frequency => 0.5,
:update => “live-preview”,
:url => { :action => “get_invoices” } %>

That calls get_invoices (notice the observe_form part):

<% if @invoices.empty? %>

Geen rekeningen gevonden. Selecteer een andere groep of maand.

<% else %> <% @invoices.each do |invoice| %> <% next if invoice.total_amount == 0 %> <% @invoice = invoice %> <% end %>
Naam Betaald
<%= invoice.parent.fullname %> <%= in_place_checkbox(:invoice, :paid, :checked => true, :unchecked => false) %>
<% end %>

A checkbox is shown in the view, but it is not working.

Does anybody knows what is wrong? Or does anybody have a solution that
is actually working?