Slot event call "When slot contents change" calls before change

In the GUI creator, when you add a slot, one of the event calls is titled, "When slot contents change." I'm trying to make it so that it tests what item just got put into the slot, but instead of returning what item is now in it, it returns what item I just took out (or if I put it into an empty slot, it returns the air before the new item). Is there something I can do to fix this? Thank you.