That's... odd. Then again, Childermass also had a distinct non-reaction to his magic. He doesn't really require any kind of negativity (for once), but he's come to somewhat expect it...

Mmmm.

"... no one... has experience with necromancy," he says very slowly. "Besides me."

He'd been under the impression that Strange did--the man had actually more or less said as much--but if so, it's such a rudimentary grasp that he was trying to kill an undead construct with a plain knife.

So the only necromancer here is Foster. And he's not looking to learn more necromancy, he's looking to learn... whatever else might exist, and prove useful to Lambert him.