diff --git a/src/fix_numdiff.cpp b/src/fix_numdiff.cpp
index c2aafa22f8..75dbf738d5 100644
--- a/src/fix_numdiff.cpp
+++ b/src/fix_numdiff.cpp
@@ -16,9 +16,8 @@
 ------------------------------------------------------------------------- */
 
 #include "fix_numdiff.h"
-#include <cstring>
 #include <mpi.h>
-#include <string.h>
+#include <cstring>
 #include "atom.h"
 #include "domain.h"
 #include "update.h"