skyrim, volkihar ruins - filling the portal vessel

[weeve9]

hi, i'm in Valerica's lab. i've read her journal, picked up the 3 required items, two i can place in the portal vessel, the 3rd item - soul void shards, when i try dropping them into the vessel i get the message "you cannot equip this item". i've tried reloading the game and going through the same sequence and each time the gem shards will not drop into the vessel. please help, tx much, rob