I don't know why, but people in Skyrim keep telling me that I have a follower while mine is dead for a long time ago. Since now, it wasn't a problem. But then I bought Dawnguard's add-on... I have to be alone when Serana takes me to the castle for the quest "Chasing Echos". She tells me "Well, I don't just take anyone into the secret parts of the place I grew up. Get rid of this interloper and you and I can be on our way". I don't know what to do... (Xbox 360)