Codeception - click simple link

How I can click on simple link <a href="/some_page.html">Some page</a> via Codeception AcceptanceTester man?

class_name: AcceptanceTester
modules:
    enabled:
      - WebDriver
      - \Helper\Acceptance
    config:
      WebDriver:
          url: 'http://mysite.local'
          browser: 'firefox'
          windowSize: '1024x768'

I'm trying:

public function somePage(AcceptanceTester $I)
{
    $I->amOnPage('/');
    $I->click('Some page', '//a[href="/some_page.html"]');
}

but getting next message:

...
CSS or XPath element with '//a[href="/some_page.html"]' was not found.
...