Mixpanel Initialize Error: argument passed to call that takes no arguments

I have a very simple question, with hopefully a very simple solution.

Mixpanel's official documentation says to initialize in 'didFinishLaunchingWithOptions' with:

Mixpanel.initialize(token: "MIXPANEL_TOKEN")

When I put it in there with my token, I get this error: "Argument passed to call that takes no arguments"

 func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    Mixpanel.initialize(token: "MIXPANEL_TOKEN")

This was working just fine a couple months ago, but now it's not. How can I fix this and why is this not working?! Thank you!