diff --git a/serpent.h b/serpent.h
index 4f76e056..c8a5a974 100644
--- a/serpent.h
+++ b/serpent.h
@@ -54,4 +54,4 @@ typedef Serpent::Decryption SerpentDecryption;
 
 NAMESPACE_END
 
-#endif
+#endif  // CRYPTOPP_SERPENT_H
diff --git a/serpentp.h b/serpentp.h
index b0053c17..8df3cb8a 100644
--- a/serpentp.h
+++ b/serpentp.h
@@ -1,5 +1,8 @@
 // private header for Serpent and Sosemanuk
 
+#ifndef CRYPTOPP_SERPENTP_H
+#define CRYPTOPP_SERPENTP_H
+
 NAMESPACE_BEGIN(CryptoPP)
 
 // linear transformation
@@ -432,3 +435,5 @@ NAMESPACE_BEGIN(CryptoPP)
 void Serpent_KeySchedule(word32 *k, unsigned int rounds, const byte *userKey, size_t keylen);
 
 NAMESPACE_END
+
+#endif  // CRYPTOPP_SERPENTP_H