diff --git a/unittest/c-library/test_library_mpi.cpp b/unittest/c-library/test_library_mpi.cpp
index 36c386144c..a1780f4043 100644
--- a/unittest/c-library/test_library_mpi.cpp
+++ b/unittest/c-library/test_library_mpi.cpp
@@ -1,5 +1,6 @@
 // unit tests for checking LAMMPS configuration settings  through the library interface
 
+#define LAMMPS_LIB_MPI 1
 #include "lammps.h"
 #include "library.h"
 #include "timer.h"