[GAMEPLAY] Sion can't hit headbutting Alistar with his ult?

So I was in a game as Sion against an Alistar and I used my ult at the Alistar. When I was about to hit him, he used headbutt on me and passed right through me, not getting hit by my ult and ending up behind me. Is this intended? Shouldn't Sion hit anybody, regardless of their motion (unless they're invulnerable)?

Steps to reproduce:

1. Pick Sion and have an Alistar on the enemy team
2. Ult at Alistar
3. Have the Alistar headbutt you as you charge at him

I haven't had a whole lot of time to test since it's finals week at ERAU.