Sivir Spellshield On Aurelion Sol Starsurge

As Sivir, I would activate my spellshield to prevenmt myself from being stunned by Aurelion Sol's Starsurge. Every time, however, I would get stunned even though I clearly saw me get mana for blocking it. Perhaps his star hit my shield, proccing it, but I don't think that happened three or four times in a row.