diff --git a/src/include/h5core/h5.h b/src/include/h5core/h5.h
index 4a298a2..c247dea 100644
--- a/src/include/h5core/h5.h
+++ b/src/include/h5core/h5.h
@@ -15,9 +15,10 @@
 
 // dummy MPI calls for serial code
 #if !defined (PARALLEL_IO)
+typedef int MPI_Comm;
 #define MPI_Init(argc, argv)
-#define MPI_Comm_size(comm, nprocs) { *nprocs = 1; }
-#define MPI_Comm_rank(comm, myproc) { *myproc = 0; }
+#define MPI_Comm_size(comm, nprocs) {(void)comm; *nprocs = 1; }
+#define MPI_Comm_rank(comm, myproc) {(void)comm; *myproc = 0; }
 #define MPI_Finalize()
 #define MPI_COMM_WORLD (0)
 #define MPI_COMM_SELF (1)