Alistar can't hit leaping champions?

In a recent game, I was playing as Alistar against a Lee Sin.

As he leaped to his ward, I head-butted him... dealing no damage and no knock back.

It's very hard to replicate without a friend in a custom match, so I can't exactly isolate the problem.

That said, is it Alistar's head-butt failing to work, or is it Lee Sin with the momentary immunity? Or is this an issue non-exclusive to either champion?