PLEASE help with sessions

I've implemented sessions with a MySQL database in PHP4. I'm registering one session variable called SESSION that is an associative array. I don't unregister it until the user logs out. The problem is, once in awhile, the SESSION array is just emptied. I check the MySQL database and the data field of the relevant record is set to SESSION|a:0:{} This is different, of course, from when the user logs out and I unregister SESSION with session_unregister("SESSION"). In that case, the data field is NULL. PLEASE someone tell me why the SESSION array is getting emptied??? Ted