Thanks you for debugging and submitting this code suggestion.
I am a noob to Piwik, but have just installed this on a Joomla 2.5 site, with a Joomla plugin to add tracking code.
When I viewed the visitor log, I too saw the same message and thought it maybe to be with Joomla, however, I followed your advise and added that line of code, uploaded to server and refreshed the visitor log page and bingo!! URL is now displayed.