not sure how i achieved this today 
Discussion
lol, i had this recently as well. I think it was on for almost 2 hrs I was out and started indicating negative. you can delete it so that it doesn't disrupt your weekly count but either way you miss out on the actual IU count. Not sure why it happens though. too much UV? :)
Need a garlic necklace and some holy water to keep the D Vampires away