dual backstab damage type

Was going through old logs and noticed this. The damage type of the second backstab on a successful dual backstab is either randomly chosen between primary wield and off hand -or- is based on the weapon in main hand. To reproduce: 1. Wield a mace or sword in main hand that the target is NOT immune to. 2. Wield a dagger in off hand that target IS immune to. 3. Backstab target. First backstab does no damage ("target is immune to your attack") but the 2nd backstab can potentially do damage.