diff --git a/src/fix_numdiff.cpp b/src/fix_numdiff.cpp
index 7d8d617b3e..ee6f8b648f 100644
--- a/src/fix_numdiff.cpp
+++ b/src/fix_numdiff.cpp
@@ -16,6 +16,7 @@
 ------------------------------------------------------------------------- */
 
 #include "fix_numdiff.h"
+#include <cstring>
 #include <mpi.h>
 #include <memory.h>
 #include "atom.h"