It can be pretty intimidating to be around The Duchess of Cambridge, which Downton Abbey star Joanne Froggatt learned the hard way.

When Duchess Kate came to visit the cast on set to watch them film a scene, Froggatt accidentally told the royal a sex joke.

During her appearance on Conan, Froggatt explained, “We're doing this scene and she's very beautiful, lovely. She has this amazing aura when she walks in the room…before we actually said hello we had to act right in front of her."

Froggatt, who plays Lady Mary on the show, said once the scene wrapped she got a chance to speak with the Duchess and she got a little bit too nervous for her own good.

When Duchess Kate told her it was strange to be in the bedroom of a character she watches on TV, "I said, ‘Yeah, not many people get to come in Lady Mary's bedroom.’ And then I was like, ‘Oh my god. What have I just said?' I've just said something that could be taken really the wrong way…and I thought, ‘Do I say? Do I draw attention to it?'"