Before calling MPI_Finalize, I am calling MPI_Comm_free at both ends (processes) of an intercommunicator created by MPI_Intercomm_create and am crashing with the following error in one of the processes: Assertion failed in file ../mpich-4.1.2/src/mpid/ch3/src/ch3u_request.c at line 480: /opt/mpich/lib/libmpi.so.12(+0x2d3bf3) [0x7f6d10e1fbf3] /opt/mpich/lib/libmpi.so.12(+0x225124) [0x7f6d10d71124] /opt/mpich/lib/libmpi.so.12(+0x33016) [0x7f6d10b7f016] /opt/mpich/lib/libmpi.so.12(+0x29eada) [0x7f6d10deaada] /opt/mpich/lib/libmpi.so.12(+0x1dfb7c) [0x7f6d10d2bb7c] /opt/mpich/lib/libmpi.so.12(+0x1d78cd) [0x7f6d10d238cd] /opt/mpich/lib/libmpi.so.12(PMPI_Comm_free+0xe7) [0x7f6d10bfe6b7] /home/kmccall/Needles2/NeedlesMpiMM(_ZN7needles12IntercomList20handleFailedIntercomERSt17_Rb_tree_iteratorISt4pairIKiPNS_16IntercomListElemEEEiPKc+0xae) [0x4c348a] /home/kmccall/Needles2/NeedlesMpiMM(_ZN7needles12IntercomList19disconnectIntercomsERilb+0xcf) [0x4c36fb] /home/kmccall/Needles2/NeedlesMpiMM(_ZN7needles16NeedlesMpiMaster8finalizeEv+0x25f) [0x496e25] /home/kmccall/Needles2/NeedlesMpiMM(_ZN7needles16NeedlesMpiMaster4loopEv+0x74e) [0x49355a] /home/kmccall/Needles2/NeedlesMpiMM(main+0x4af) [0x48d1e5] /lib64/libc.so.6(__libc_start_main+0xf3) [0x7f6d0f186ca3] /home/kmccall/Needles2/NeedlesMpiMM(_start+0x2e) [0x48cc7e] internal ABORT - process 0 I looked in ch3u_request.c and there seems to be no assertion at line 480. If I forgo the MPI_Comm_free in the first process, the same error then occurs in the second process. Does this give you any clue as to what I am doing wrong? Thanks, Kurt