【piwik】What is “page url not defined” in the Pages report?

Piwik provides two reports for your page views: Page Titles and Page URLs. In some rare cases, a visitor’s browser will not let the Piwik Tag request the current URL (generally for privacy reasons). If Piwik could not request the page URL from the browser, Piwik will instead report “page url not defined” for the visits on those pages.