Wreath of Phrecia not equippable on single-stat mercenaries

The unique helmet Wreath of Phrecia explicitly has No Attribute Requirements, so it should be equippable on any merc like belts, rings and amulets. It is equippable on a Dex/Int Bitterblade, but not an Int Toxicologist.

I need to prevent my merc from poisoning, please send help! :)

Thank you for your time!

Bump. This bug still unsolved from start of the league, as is wands Hunter mod "Curse Enemies with Despair on Hit". According to the info on poedb, this modifier is no different than the one on the rings. And yet it doesn't work.