Nocturne Spell Shield NOT blocking spells

Multiple times in several games, while playing Nocturne, I undoubtedly had my spell shield up and still took damage/cc from a spell. The attack speed passive would proc as usual but the damage still applied. One champion I know for a fact this happened against was Lucian, when he dashed and fired as per the usual combo. The other was a Jarvan E-Q combo. I know the flag didn't hit me because I was attempting to peel for a carry. Not sure if this is as intended, but it does say spell shield, which I understand to be any cast ability.