Can't Set Sims on Fire?

So, I'm playing Sims 4 (all packs) and need some fire ghosts. I'm using my go-to method: making a small room, putting the Sims to be killed in the room, putting a fire ghost inside with them, making the fire ghost angry, having them set a fire on the floor. But, whatever I do, my Sims WILL NOT catch fire! At all! They'll be standing right beside it while it spreads, but it completely avoids them. I tried putting rugs in the room and having my Sims stand on them, nothing. I'm getting really frustrated because I want to start actually playing this particular save file but I can't until I get these ghosts made.

I don't know if it matters, but I keep having to stop my Sims from putting out the fires (free will is turned off, but they still try to put them out). But there are always still flames around so I doubt that matters.

Any idea how to set them on fire? And, before anyone suggests it- I can't just download fire ghosts from the gallery because I need the ACTUAL ghosts, with tombstones, for my story.