Problem with assign in parallel

Hello,

I am working with an advection-diffusion problem that works fine when running it in serial. However, when running it in serial with the following command:

mpirun -n 6 python NSfracStep.py problem=TayCyl

It the following error:

*** Error:   Unable to successfully call PETSc function 'VecCopy'.
*** Reason:  PETSc error code is: 73. (Object is in wrong state).
*** Where:   This error was encountered inside /build/dolfin-fyfCiA/dolfin-2017.2.0.post0/dolfin/la/PETScVector.cpp.

in this line:

 cp_n.assign(cp)

in which cp is the solution function that is just solved, which I am assigning to cp_n which is the solution of the previous timestep.

Does anyone know how to fix this problem in parallel?