Yoricks Shepherd of souls

So curious are Shepherd of soul minions suppose to be taking 0 damage from abilities both AP and AD ? cause noticed it with nasus.. irelia swain and a few others that when you use abilities on shepherd of soul minions you do minimum to 0 damage to them while other minions seem to take damage for abilities yoricks don't

not sure if this is a bug that is needing to be reported

or if this is actually designed cause it does make it extremely hard to fight yoricks when you have to auto attack his shepherd of soul minions when his ult is spawning them like crazy at a tower and so on