I tried working this out, but I think the original was correct.
The cases where the answer is negative correspond to a 'runaway scenario' where every change is expected to cause more than 1 extra change. So the answer is 'nonsensical' (because that is indeed where the formula for geometric series no longer works) but the true answer is infinity.
The cases where the answer is negative correspond to a 'runaway scenario' where every change is expected to cause more than 1 extra change. So the answer is 'nonsensical' (because that is indeed where the formula for geometric series no longer works) but the true answer is infinity.