Is this a bug?

[luisr]

I have a YaBB SE (version 1.3.1) board in my site and every once in a while I get this kind of error message:

2: mysql_num_rows(): supplied argument is not a valid MySQL result resource
(/usr2/home/juanadiaz.org/htdocs/foro/Sources/BoardIndex.php ln 170)

This one appears when trying to display the forum index.  Most other functions not involving the index appear to work properly.

After some time it appears to fix itself because I do nothing to fix it.  Is this a bug?  If so, is there a fix?

Please don't tell me to update to the latest version because it is not going to happen for the time being.  A relocation to a new host is coming and I will install the latest version in the new host before relocating the entire site and transferring the database.