failed to evaluate jacobian in using velocity field in fluid particle tracing

Hi,

I have tried to edit drag force's equation view in fluid particle tracing(fpt). COMSOL for example, uses "model.input.u1-fpt.vx" for relative velocity in x direction but when i use it in my equation i encounter "Failed to evaluate Jacobian" error.

Do any one know how can i solve this error?

Thanks a lot,
Navid