Byzantium Pronoiar question

Hey guys, I've got a quick question about pronoiars.

Playing as Byz, I've had a few pronoiars and annexed them via inheritance. However, now I've vassalized Haasa but it won't let me make them a Pronoiar because they are a monarchy. However, I've made Mikhlaf a Pronoiar before which I believe is the same government system. Any ideas on what is wrong?

Quite unsure where to find the exact requirements to land a Pronoiar at all. Some help would be appreciated, thanks in advance!