BigH - there are zero calories in diet coke also, but there is more to consider than just calories. There is citric acid in diet coke which can knock you out of ketosis.
Coke zero does not have citric acid in it, it has malic acid. This will not knock you out of ketosis. As someone mentioned, all low cal soft drinks have aspartame in them and some people find they stall them. It's horses for courses, what works for you.
Technically coke zero is not allowed on SS. But if it's a case of having coke zero and sticking with SS and getting on with it, or falling off the wagon face first into the nearest pizza then of course it's better to have the coke zero than fall off the wagon.
I personally find it makes me really really hungry, although I still am in ketosis according to the ketostix. I get a really grumbly tummy and feel starving so it's not worth it as far as I'm concerned. But other people find different and it doesn't make them hungry.