[Spell] Turning Spell Shield into a unit ability

[Teelo]

I've taken the ability Item - Spell Shield and turned it into a unit ability, but now it isn't working. No buff is showing, and it is a clickable ability - clicking it doesn't do anything besides trigger the sound anyway.

I figure it must be because the workings of the ability come from the item, rather than the item ability? How could I make this work as a passive unit ability with a cooldown?