diff --git a/lib/rc-md5.h b/lib/rc-md5.h
index a30f16d..dcde619 100644
--- a/lib/rc-md5.h
+++ b/lib/rc-md5.h
@@ -10,6 +10,7 @@
 #define _RC_MD5_H
 
 #include "config.h"
+#include <stdlib.h>
 
 #ifdef HAVE_NETTLE