Page URL not defined error

I am getting error Page URL not defined in following API.
http://localhost/index.php?module=API&method=Actions.getPageUrls&idSite=6&period=range&date=2020-01-03,2020-03-06&format=JSON&token_auth=aaba8c30fb8e5df0b5c66654ac796c2f&expanded=1&flat=1&filter_limit=-1

Please help me to understand how a page url can be undefined? Or why matomo mark a document URL as undefined?
Thanks and Best Regards!