Strange line in viewfinder on D5000

I noticed that when the center focus point locks on a subject a very thin vertical line up from the center of the viewfinder appears. It doesn't do this for any other focus point. It's not really a big deal since it doesn't show up in pictures, it's just a bit distracting. Anyone have an explanation?

I have D5000 But I haven't see any kind of center line. I have been using This one since 1 years and now. It is working well till now.