problem date_select with active_form

hi,
i have a problem with date_select and and active_form plugin

example:

view:
form_for( "contact", :url => { :action => "form_contact" } ) do |f|
.... other code ....
f.date_select :period, :order => [:day, :month, :year]
.... other code ....
end

model:
class FormContact < ActiveForm
attr_accessor :name, :surname, :period, ......
... validations
end

control:
def form_contact
@contact = FormContact.new(params[:contact])
respond_to do |format|
if @contact.valid?
..........
end
end
end

when i submit the for wen is present the date_select i retrive an error
controller like this:

`@period(3i)' is not allowed as an instance variable name

Where is that error appearing, is it your code or some plugin? If it
is your code what do you expect @period(3i) to do?

Colin