Here is a screenshot of the error you receive when trying to socket the item.
imgur . com/a/aaJ9610
And just incase someone says that offhands cannot be socketed, please check out this priest (https://worldofwarcraft.com/en-gb/character/eu/silvermoon/stylebenderx) and see what offhand is using. The same one I am trying to socket, except he did use the right sequence (socket → upgrade).
Please read this thread about the same issues posted in the Arena section You can't socket an item after upgrading it with a trophy - #21 by Iwnlx-outland