I have the occasional Coke Zero too.
But be careful if you're having other diet drinks. I think Dr Pepper Zero is also okay because there's no citirc acid in it - but citric acid can interfere with ketosis - in other words, your body may not be able to break down fat and use ketones for energy while there's citric acid knocking about. Your body will have to burn something though, and as we're not eating many carbs, it'll probably burn muscle instead. Not good. You'll still lose weight - but it'll be muscle you're losing, not fat.
The odd can here and there when you're desperate and can't get your hands on something 'safe' won't do you too much harm, I don't think. But it's probably wise to restrict consumption of citric acid as much as possible.