Topic: posting with mechanize

I am using 0.9.3 mechanize and  and I want to post data but  radiobutton  and  checkboxes is getting error. Please help.
my app is so simple : page = agent.post('http://www.example.net/', {
:radio1=> agent.radiobuttons_with(:name => 'checkbox1')[2].check, # error
:checkbox1=> agent.post_form.checkboxes.name('checkbox1')[2].check, # error
:IMAGE5=> File.open("#{RAILS_ROOT}/Sunset.jpg",
:spec=> 'test' # checkbox
})

why?