Setting session var to null now wrong?

I recently noticed that a commit late on Oct 2nd to session.c makes setting a session variable to null now incorrect and spits back a warning at the end of the page. I did use the ini workaround to get around this, but I am still wondering why it is wrong to do this. I am setting session variables via output from my database which sometime does have a null field (no global vars used). Why (not how to get around it) is doing this wrong? In addition, why should it spit out a warning on every page that uses sessions rather then just the page that tried to set the variable? Just Wondering, --Matt