But fans couldn't really get annoyed once the actress (who is only 13!) took to Insta to apologise and explain why she couldn't be there.
"Hey guys, I've never had to do one of these videos before. But I've had to cancel at Collective Con last minute, which is something I've never done and I'm planning on never doing again," she says in the heartbreaking video.
She added: "I just think I've worked too hard and I have to rest as I've had a really long shoot and I'm still filming Stranger Things. I'm sorry to everyone who's going and I promise you guys I'm going to get back to you. I love you guys all, thank you so much for your continued support and thank you."
Poor Millie! Rest up and we hope you feel better soon!
This article previously appeared on WHO.
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