use observe_field within form_tag

Hey,

I'm trying to observe a field that is inside a form_tag like so:

<% form_tag 'send_mail' do %> <%= select_tag 'select_club', options_from_collection_for_select(clubs,'id','name'), :class => 'ComboBox' %>

<%= observe_field 'select_club',:url=>{:action => 'clubs_changed'} %> <% end %>

When I run this in FireFox gives me the following JavaScript error: Form is not defined and then points to the generated javascript: new Form.Element.EventObserver('select_club', function(element, value) {new Ajax.Request('/mailings/clubs_changed', {asynchronous:true, evalScripts:true, parameters:value + '&authenticity_token=' + encodeURIComponent('06f49bb619c38f8d0ea988932e4324abbd8511ef')})})

My html does contain a form so I don't exactly know where to look anymore.

Thanks!