Hm, this isn't terribly helpful - we need to actually see it in context. An excerpt of code just in the <head> doesn't do it. For example, are you using the ES Module CustomEase.js file or the ES5 one? Is there a reason you're not using the minified one?
If you still need some help, please provide a reduced test case (ideally in codepen). If you're not sure how, see: