Hello, Taking http://www.mcs.anl.gov/petsc/petsc-current/src/vec/vec/examples/tutorials/ex... and changing its 56th line from VecSetValues(x,1,&i,&one,ADD_VALUES); to VecSetValues(x,0,NULL,NULL,ADD_VALUES); yield the following error (at least --with-debug=1): [0]PETSC ERROR: --------------------- Error Message -------------------------------------------------------------- [0]PETSC ERROR: Invalid pointer [0]PETSC ERROR: Null Pointer: Parameter # 3 What is the rationale behind first checking the pointers, then doing the work or lack thereof, instead of, first checking if there is some work to do, and only then check the pointers (à la MPI) ? Thank you for your help, Pierre