I am trying to use the Schur complement preconditioner in petsc, but am encountering a null argument error calling PCFieldSplitGetSubKSP.

This only happens on PC_COMPOSITE_SCHUR, the multiplicative/additive options do return a KSP array.

Error message:

[0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
[0]PETSC ERROR: Null argument, when expecting valid pointer
[0]PETSC ERROR: Null Object: Parameter # 1
[0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
[0]PETSC ERROR: Petsc Release Version 3.6.0, Jun, 09, 2015
[0]PETSC ERROR: ./elecmech on a arch-linux2-cxx-debug
[0]PETSC ERROR: Configure options --with-shared-libraries=1 --with-debugging=1 --download-openmpi=1 --with-clanguage=c++ --download-fblaslapack=1 --download-scalapack=1 --download-blacs=1 --download-suitesparse=1 --download-pastix=1 --download-superlu=1 --dowload-essl=1 --download-ptscotch=1 --download-mumps=1 --download-lusol=1
[0]PETSC ERROR: #1 MatSchurComplementGetKSP() line 317 in petsc-3.6/src/ksp/ksp/utils/schurm.c
[0]PETSC ERROR: #2 PCFieldSplitGetSubKSP_FieldSplit_Schur() line 1264 in petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c
[0]PETSC ERROR: #3 PCFieldSplitGetSubKSP() line 1668 in petsc-3.6/src/ksp/pc/impls/fieldsplit/fieldsplit.c

Code snippet:

ISCreateGeneral(PETSC_COMM_SELF,xp_dofs,&ii[0],PETSC_COPY_VALUES,&is_xp);
ISCreateGeneral(PETSC_COMM_SELF,all_dofs - xp_dofs,&ii[xp_dofs],PETSC_COPY_VALUES,&is_wk);

PCSetType(pc,PCFIELDSPLIT);
PCFieldSplitSetType(pc,PC_COMPOSITE_SCHUR);
PCFieldSplitSetIS(pc,"xp",is_xp);
PCFieldSplitSetIS(pc,"wk",is_wk);
int n;
KSP* ksps;
PC   subpc;
PCFieldSplitGetSubKSP(pc,&n,&ksps);

ii here is simply an array with ii[i] = i. There is probably a better way to simply indicate two blocks of different size, but I couldn't find it.

Thanks,

Sander