Conductive Media DC(shell) + Electrostatics

hello

I have been trying to couple the two application modes to find the electrostatic force on one boundary at a fixed voltage due to the voltage on a parallel boundary which has a variable voltage along it due to current flowing in it.I am using comsol 4.1.I keep getting the error
"Constraint found for variables in different Segregated groups. Try to merge these groups."

I thing this basically means both the applications are trying to set the voltage along the current flowing boundary.I think so because when I dont link the DC Shell output voltage to Electrostatics I do not get this error.Does anyone know how to get around this or know any other way to calculate this force.I am using Structural Mechanics module,MEMS module, and AC/DC module.

Any comments are appreciated.

thank you