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):

<h1>Verwerken rekeningen</h1> <%= start_form_tag({:action => "get_invoices"}, :id => "entry-form") %>   <p><label for="invoice_month">Maand</label><br/>   <%= select_year(Date.today,                   :start_year => Schoolyear.get_startdate.year,                   :end_year => Schoolyear.get_enddate.year)%>   <%= select_month(Date.today)%></p>

  <p><label for="parent_payment"><%= Parent.human_attribute_name("payment")%></label><br/>   <%= select(:parent, :payment, %w{ 6 2 1}) %></p> <%= end_form_tag %>

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

<div id="live-preview"></div>

That calls get_invoices (notice the observe_form part):

<% if @invoices.empty? %>   <p>Geen rekeningen gevonden. Selecteer een andere groep of maand.</p> <% else %>   <table>     <tr>       <th>Naam</th>       <th>Betaald</th>     </tr>    <% @invoices.each do |invoice| %>     <% next if invoice.total_amount == 0 %>     <% @invoice = invoice %>     <tr>       <td><%= invoice.parent.fullname %></td>       <td><%= in_place_checkbox(:invoice, :paid, :checked => true, :unchecked => false) %></td>     </tr>   <% end %>   </table> <% 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?