Why does Orlok erupt with such anger when Thomas asks about the Romani people staking the corpse?

Thomas asks Orlok about some of the things he witnessed in the village on his way to Orlok's castle. He mentions the Romani people opening a grave and staking the corpse. This is a filthy ritual, according to Orlok. When Thomas asks about what manner of ritual it is, Orlok angrily tells him not to speak of it again. Does the fact that there are people that know how to deal with vampires upset Orlok? Does he feel threatened in some way? That is the impression I got, but I'm curious to hear your thoughts.