Merc cannot use weapons with "hel" rune

This is the second time I’m encountering this bug and I reported it before but I got no response.

This time I’m trying to equip a Cryptic Axe “obedience” on my merc. He equips it, but it instantly turns red and his damage is the same as when he has no weapon. (yes, he meets ALL the requirements listed on the weapon)

I believe this a bug with the “hel” rune giving -20 requirements. Because other mercs who have higher stats can use it.