Capybara cannot find date picker
===

Using capybara, I am trying to fill in a date. There is a javascript date picker on the form. I have tried the following ways to choose it:

    find('input[type=date]').focus()
    find(:xpath, ".//*[@id='Sdate']")
    fill_in('Sdate', :with => '25/07/2555')

However for all of these attempts, I got the exact same error message:

    Selenium::WebDriver::Error::UnknownError: '[JavaScript Error: "a is null" {file: "file:///var/folders/ky/r1ft86wd5x9cjq0f3f6tq0600000gn/T/webdriver-profile20120725-41019-m72bh5/extensions/fxdriver@googlecode.com/components/command_processor.js" line: 5813}]' when calling method: [nsICommandProcessor::execute]

I am using Selenium webdriver on Firefox with Capybara 1.1.2 in a Rails 3.2.6 task. 

What am I missing?