When patch 1.01 arrived I HUNGERED for the new potion to remove a random injury. So I started building Conclaves to get to Apothecary. But for some reason I couldn't build Apothecaries in some cities. As it turns out, the city has to have at least 1 essence in order for me to build an Apothecary.
Now, the main function of an Apothecary is something that scales with Essence, so I can understand why the requirement is there.
But the secondary function of an Apothecary is just enabling a bunch of potions in the city, and with that in mind it doesn't make sense to require Essence to build the improvement.