robot causes session_start() error?

[necron]

Hi guys,

I noticed that once in a while I get this range of session_start() error messages in my php error_log file. They all happen in a short period of time (about half an hour) and many of them within 3 or 4 minutes difference:

session_start(): The session id contains invalid characters, valid characters are only a-z, A-Z and 0-9 in [...] on line 90

I checked the website and cannot recreate that error unless I manually type an incorrect session ID into the URL field.

I was wondering whether it might be a search engine robot that causes this problem by entering the page with an incorrect URL string?

Have you got any recommendations on how to stop this error from occuring?

Thanks heaps!