The issue should be fixed with 4.2 enabled

This seems TS 3.3 specific. If you switch to 2.4/ES6, it should work fine.

Not an issue, the natural there is to limit the range of numbers to be considered to be at least 0 ;-)

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Finish the solution so that it returns the sum of all the multiples of 3 or 5 below the number passed in.

There are no natural numbers below a negative integer, the answer then is 0.

Negative integers aren't natural numbers by the generally-accepted mathematical definition.