Shield Confusion

Basically, I main Lux. I play her midlane as much as possible but there is one issue I noticed several times when I played against Ahri. I noticed that everytime my shield was up, Ahri's charm would disappear and not have any effect on me whatsoever. But this only happens if I'm shielded. If I'm not, I get charmed. My allies, even with the shield also get charmed, it does not seem to work for them. Is this meant to happen or just a bug because this happens everytime I'm against an Ahri. Some advice would really help. Thanks!