Necromancer – “Ruler of Erathia” [Campaign 3, Mission 5]

I don't understand why I can't turn people into undead if I have that building constructed in the Necropolis castle. They turn into undead perfectly fine, even when they are in the hero's party – a Knight, for example. I just need to bring them to the castle. It doesn't cost any money. It's probably just a bug.

I received the "Subdue" spell in the Pyramid during the third mission.