Error on certain condition

First of all, this is a great module. While i kepp using this module to communicate with users sponatenously. But, sometimes there are errors that appear like this:

user warning: Got error 28 from storage engine query: SELECT * FROM pm_index i LEFT JOIN pm_message m ON m.mid = i.mid WHERE (i.uid = 18 AND i.is_new = 1) AND i.mid NOT IN (SELECT mid FROM pmgrowl_close WHERE uid = 18 and closed = 1) ORDER BY m.timestamp ASC in /home/kripkorn/domains/mysite.com/public_html/sites/all/modules/pmgrowl/pmgrowl.module on line 87.

There are not one. Sometimes it grow about 5 same error occur. When i look in recent log entries, i found that it most probably came from GUEST. When guest, like Google bot try to crawl all the site, it also have permission to accidently access this module.

So, what i try to suggest is to add on permission based on the role. By that we can block accessing this module for anynomous users.