Your code fails for [] and [true] because if you perform AND on true and undefined, it will result in undefined

I ran into the same problem. Update sum with the following code in place of yours:

sum = int(sum//2)

Yes, that's TWO division signs. This returns a result rounded to the nearest whole number. You need that or else the new sum value will throw off certain iterations because of the remaining decimal value.

I never knew about the // operator until looking stuff up for this kata.