Legendary Refining Stones

Hey Guys

I have a question about the Legendary Refining Stone.

I get one Armor Legendary Refining Stones over the daily reward. I was happy because i have re roll my karasha shield but i cant use the armor stone. The shield is +15 upped, when i try to choose the stone i got the message "invalid item".

Question

Should i use a weapon legendary refining stone for my shield?