That query is only used in comshow.php
I would suspect this is a BOT issue.
That error would only happen if you have no comments and in our software we do not show a link to users unless there is a comment so bots have to be scanning different scripting.
I can alter the query to surpress any error by simply putting '' around the id variable however my suggestion would be to put this in the robots.txt file in your classifieds folder
Code:
Content visible to verified customers only.