When you observe the error feedback it tells you that your function returns NaN, so you should start debugging with console to see what is what and why that happens in the first place.

Specifically, look at what lastIndex amounts to, and what results from that.

Good luck!

First of all, please do not post as an issue when you do not know why your code does not pass. That's reserved for when there's a problem with the kata code and you can give examples as to why. Instead, post as a question.