Aurelion Sol's Skills ignore Sivir's Spellshield

I played Sivir today and noticed that sometimes Aurelion's skills ignored my spellshield. At first I thought it might just be me, but then suddenly he stunned me, but my spellshield was up, but did not get triggered. If I had been to slow, the stun would've prevented my spellshield from activating, therefore I assume there is a bug.

Thanks.