Not A Bug Subtlety broken?

[Firedrake]

Searched for any posts on Subtlety, but none found, so...

I flashed in Subtlety when my opponent was attacking with 3 creatures. Got a message that there were no legal targets. There was nothing on board giving anything hexproof or shroud. I clicked on one of them anyway, to no avail.

 

[Torbin]

Tag applied: Needs "Not a Bug" tag.

Testing applied: Recreated gamestate by casting Subtley while being attacked by 3 creatures.

Test Results: Subtley's trigger only applies to spells being cast that are on the stack and not permanents already on the battlefield.