Sometimes it happens that the sorc pets are invisible and therefore untargetable but you still can use their skills and they do damage.
I have no idea how you can manage to do this but i had this bug also myself often but i assumed my pet got stuck somewhere but today i faced an enemy player and he seemed to have the same bug but i still took damage from his twillight.
Ice Furnace: This item set now grants Spell Damage, rather than Weapon Damage for the 4 piece bonus
- Update 23