diff --git a/src/message_passing.cpp b/src/message_passing.cpp
index 3ab96ffd8..2ff3952d7 100644
--- a/src/message_passing.cpp
+++ b/src/message_passing.cpp
@@ -3,8 +3,8 @@
 namespace openmc {
 namespace mpi {
 
-int rank;
-int n_procs;
+int rank {0};
+int n_procs {1};
 
 #ifdef OPENMC_MPI
 MPI_Comm intracomm;