dual backstab is checking damage type of non-dagger

I'm wielding a mace in main hand and dagger in off hand. The zombie is immune to the attack type of my primary weapon, but not my dual-wielded dagger. It seems like the dual backstab goes through, but does no damage because the zombie is immune, which makes me think it's incorrectly checking the damage type of my mace. Your backstab <<< ERADICATES >>> the juju zombie! The juju zombie is unaffected by your second backstab!