You have to enter codes at least every 90 days to be considered active, i believe...
Just the other week (I thought that I had been), I received an email that my points were about to expire. But due to my work schedule, I didn't get a chance to log in until the expiration date. Not wanting to lose my 300 and some points, though, I immediately called them and told them the problem. Customer Service were great -- and immediately put the points back in again!
FE: What do you mean that you go negative by 17?