Translation:Could you type that letter for us?
I understand, but they do have slightly but certainly different meanings.
Re-reading my post, I realize that I didn't mention that this was a listening exercise. With listening exercises, I'm often well aware that when I hear "mi" it could be "ni" (and vice-versa), and so I end up listening to the sentence several times, trying to figure out which word is being spoken.
So when I take such care to get "mi/ni" right, but end up getting it wrong, it's rather frustrating for me (and I suspect for other people as well).