Ruby-on-rails – Rails simple form gives InvalidAuthenticityToken error

I have a simple form like this:

<form name="serachForm" method="post" action="/home/search">   
  <input type="text" name="searchText" size="15" value="">
  <input class="image" name="searchsubmit" value="Busca" src="/images/btn_go_search.gif" align="top" border="0" height="17" type="image" width="29">
</form>

And a controller with this method:

  def busca
    puts params[:searchText]
  end

When I do a click on the image button in the form I get a ActionController::InvalidAuthenticityToken. here's the full StackTrace:

/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/request_forgery_protection.rb:86:in
verify_authenticity_token'
/Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/callbacks.rb:178:in
send'
/Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/callbacks.rb:178:in
evaluate_method'
/Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/callbacks.rb:166:in
call'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/filters.rb:225:in
call'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/filters.rb:629:in
run_before_filters'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/filters.rb:615:in
call_filters'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/filters.rb:610:in
perform_action_without_benchmark'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/benchmarking.rb:68:in
perform_action_without_rescue'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/benchmarking.rb:68:in
perform_action_without_rescue'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/rescue.rb:136:in
perform_action_without_caching'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/caching/sql_cache.rb:13:inperform_action'
/Library/Ruby/Gems/1.8/gems/activerecord-2.2.2/lib/active_record/connection_adapters/abstract/query_cache.rb:34:in
cache'
/Library/Ruby/Gems/1.8/gems/activerecord-2.2.2/lib/active_record/query_cache.rb:8:in
cache'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/caching/sql_cache.rb:12:in perform_action'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/base.rb:524:in
send'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/base.rb:524:in
process_without_filters'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/filters.rb:606:in
process_without_session_management_support'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/session_management.rb:134:in
process'
/Library/Ruby/Gems/1.8/gems/actionpack-2.2.2/lib/action_controller/base.rb:392:in
process'
/Library/Ruby/Gems/1.8/gems/rails-2.2.2/lib/webrick_server.rb:74:in
service'
/Library/Ruby/Gems/1.8/gems/rails-2.2.2/lib/commands/servers/webrick.rb:66 /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:153:in
require'
/Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:521:in
new_constants_in'
/Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:153:in
require'
/Library/Ruby/Gems/1.8/gems/rails-2.2.2/lib/commands/server.rb:49

What is happening?