Infinite rebirth, finite kamma?

If we have been transmigrating in samsara infinitely via rebirths, then doesn't that mean that kamma which we have earned too is infinite? And if so, doesn't that imply that enlightenment is imposible to reach since it would take one infinite amount of time to reach? I know this is classified as something that is unconjecturable and drives one insane, but this one is driving me curious. Any takers on this?