Error in DEP simulation-Help

Hi All,
I m getting simulation error while simulating DEP based separation.I choose particle tracing with time dependent study, I m able to simulate for laminar-starionary, electric field-stationary..
i have following error when simulating particle tracing - time dependent study
Complex number encountered in real expression.
- Expression: (mod1.fpt.Ftx-d(mod1.qx,t)*d(mod1.fpt.mp,t))/mod1.fpt.mp
- Geometry: 2
- Boundary: 1

If anybody can help, i will be highly oblige