Gameplay; Sivir's Spell Shield does not block Gnar's Boulder Toss

I was playing in an ARAM game against a Gnar as Sivir, and as the title suggests my spell shield did not block the damage nor the slow from Gnar's boulder. It happened three times in my game and I died to it once (lol).

That's the first time I experienced this. I also don't have a video of this because I don't have the necessary software/knowledge...

If you need any other information, just ask me and I'll answer the best I can! Hope this helps!