diff --git a/md5.h b/md5.h
index 7b55d01..90d5ec4 100644
--- a/md5.h
+++ b/md5.h
@@ -4,6 +4,7 @@
 #define _CRT_SECURE_NO_WARNINGS
 
 #include <string>
+#include <cstring>
 
 std::string md5(std::string dat);
 std::string md5(const void* dat, size_t len);
@@ -12,4 +13,4 @@ std::string md5file(std::FILE* file);
 std::string md5sum6(std::string dat);
 std::string md5sum6(const void* dat, size_t len);
 
-#endif // end of MD5_H
\ No newline at end of file
+#endif // end of MD5_H