Handle error case for unknown MPI
@@ -73,6 +73,8 @@ static char python_parallel_error_string[4096];
static char* mpi_impl = "MPICH";
#elif HAVE_MPI_IMPL_OPENMPI
static char* mpi_impl = "OpenMPI";
+#else
+#error "Must specify MPI_IMPL!"
#endif
#define EXCEPTION(ee) \
