diff --git a/m-string.h b/m-string.h
index b71edff9..8ad61255 100644
--- a/m-string.h
+++ b/m-string.h
@@ -28,6 +28,7 @@
 #include <ctype.h>
 #include <stdio.h>
 #include <stdarg.h>
+#include <string.h>
 
 #include "m-core.h"