word choice - What's the difference between まで and までに?

I'm studying for N3 Japanese test. I'm stuck in a sentence of an exercise in which is written

帰国{きこく}するまでになんとか日本語{にほんご}を習得{しゅうとく}したい。

Why is it wrong to say

帰国するまでなんとか日本語を習得したい。?

I really don't get it, if someone could teach me the difference I would be really happy. Thanks!