Seems you have an addon that changes IsEquippedItem, more exactly sets that API to nil, so it does not exist.
http://wowprogramming.com/docs/api/IsEquippedItem
You can if you have, search all your .lua files in Interface\Addons and see what addons use that API, by searching for "IsEquippedItem" -maybe you find something saying "IsEquippedItem = nil" or similar.