CC pet bath only works when non CC bath is also on lot?

I just downloaded a CC pet bath for dogs. The bath is only designed to be used by dogs, not Sims. After some testing I realized that it only works if a non CC bathtub is also on the lot. It won't work on its own, I get the message: "All available bathtubs are clogged." I think the file is quite old, because it was uploaded in 2017.

I can work around this limitation, but does anyone know why it's doing this? I'm curious about the technical aspect.