Quality factor for piezo device in 4.3

Hi all-

I'm trying to calculate the quality factor for a piezo mems device in 4.3, using the piezo physics interface. Whenever I try to calculate it, I get an error saying that it's unable to calculate pzd.Dx2 or pzd.Dx3. I for the life of my can't figure out what these two variables are. pzd.Dx I've got, but what are these two, and does anyone know what would make it unable to evaluate them?

Thanks as always,
Kamala