ERROR: Complex number encountered in real expression(when calculate the DEP force)

Hello everyone:
I use Particle tracing module for fluid to calculate the DEP force exerted on the nano-sized particles.

When calculate the DEP I encountered a problem as follows:

Complex number encountered in real expression.
- Expression: (mod1.fpt.Ftz-d(mod1.qz,t)*d(mod1.fpt.mp,t))/mod1.fpt.mp
- Geometry: Particles

Anyone can help me ?
Thanks!