Coke zero and ice cream effect?

I was going to make a rootbeer float, but we only had coke zero. So Im eating a coke zero float, and i found that the soda made a hard crust around the ice cream, does anyone know why?Coke zero and ice cream effect?
The coke zero is probably warmer than the ice cream so there's a hard shell of ice cream. I've had this before and I've noticed it does that with most sodas.Coke zero and ice cream effect?
Any cold beverage does that...even root beer. The liquid is warmer than the ice cream so it melts and then refreezes and creates a shell.