ci_sessions table - session data NULL

[eluser]surfgatinho[/eluser]
Whenever I check my ci_sessions table I find around 90% of all entries have session data NULL.
It doesn't seem these are cleaned up by the garbage collection and when there are nearly 100,000 rows like this it starts to cause problems.

I may well write a cron job to clean these out every day but I was wondering if this problem is something to do with my setup, or why the GC doesn't remove them