Question on lethality and magic pen.

So I get what it does, pretty obvious stuff (even tho they could have just named lethality armor penetration), what I want to know is how this works, if say, the enemy you are attacking has no armor, or no MR. Does it drop the armor or MR into negatives, or does it have no effect at all. So for example if you decide to buy a void staff and no one has MR, is it a waste?
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