What sweetener is in the sugar free options?

What it says in the title. I looked on the website, asked the AI, and sent an email, and searched this sub. I can't find anywhere what the actual sweetener is. Aspartame gives me wicked headaches, but I'm trying to keep my sugar intake low.

We have a DutchBros opening in my little town in a couple of months, and I want to "know before I go."

Thanks in advance to anyone who can help!