notToBeBetween seems to be the same as toBeBetween
You are being a bit "uncontrolled" in your catch there. You should only be catching whatever AssertionException that between might throw; you shouldn't be catching everything. Because you're also catching the exception fail throws here, and... that's yer problem! (thanks to for noticing the cause here. I missed it on first take!)
Thanks guys!. Is a PR coming?