How can I switch between two frames with Capybara

I am trying do something inside 2 frames but error raises everytime when I try to switch between frames. For example:

# encoding: utf-8

require "capybara/dsl"

Capybara.run_server = false
Capybara.current_driver = :selenium
Capybara.app_host = 'https://hb.posted.co.rs/posted'

class Account
  include Capybara::DSL

  def check_balance
    visit('/')
    page.driver.browser.switch_to.frame 'main'
    fill_in 'korisnik', :with => 'foo'
    fill_in 'lozinka', :with => 'bar'
    click_button 'Potvrda unosa'

    page.driver.browser.switch_to.frame 'header'
    click_on 'Stanje' 
  end
end

account = Account.new
account.check_balance

Error is:

[remote server] file:///tmp/webdriver-profile20120810-9163-xy6dtm/extensions/fxdriver@googlecode.com/components/driver_component.js:6638:in `unknown': Unable to locate frame: main (Selenium::WebDriver::Error::NoSuchFrameError)

What is the problem? Maybe I am doing something wrong here?

If I change order of switching frames so try first to switch to 'header' then switch to 'main' frame then same error raises except that it says that this time there is no 'main' frame:

# encoding: utf-8

require "capybara/dsl"

Capybara.run_server = false
Capybara.current_driver = :selenium
Capybara.app_host = 'https://hb.posted.co.rs/posted'

class Account
  include Capybara::DSL

  def check_balance
    visit('/')
    page.driver.browser.switch_to.frame 'header'
    click_on 'Stanje' 

    page.driver.browser.switch_to.frame 'main'
    fill_in 'korisnik', :with => 'foo'
    fill_in 'lozinka', :with => 'bar'
    click_button 'Potvrda unosa'
  end
end

account = Account.new
account.check_balance

Error:

[remote server] file:///tmp/webdriver-profile20120810-9247-w3o5hj/extensions/fxdriver@googlecode.com/components/driver_component.js:6638:in `unknown': Unable to locate frame: main (Selenium::WebDriver::Error::NoSuchFrameError)