I recently bought a nightmare potion for my pet Tepish, who I changed to a Harvester, but it keeps saying "Invalid combination" when I click the 'use on pet' button. I'm not sure what is going on and it doesn't say anything about this problem in the pets faqs. Any advice?

that means that particular color hasn't been released yet.

Because there is no such thing as a nightmare harvester. You can see all the available harvester colors HERE. Ee! That sucks, maybe they will make one someday. Sorry. :(

I'm so sorry! That pet colour is unavailable. :( Here's all the NM pets available. :)

Ahk, thank you guys! ^_^ Sucks though... :/