[It takes her a second, but she slowly starts to fluster as she grows aware of her mistake.]

O-oh, um... I see. I apologize. When we were introduced, you must have used the Western presentation of your name, then, didn't you?

I've heard it used interchangeably with someone I work for, so I'm, um... I'm very sorry if I was coming across too informal...! That wasn't my intention.