diff --git a/ndn-cpp/PublisherPublicKeyDigest.hpp b/ndn-cpp/PublisherPublicKeyDigest.hpp
index 5ce4124..95ca25d 100644
--- a/ndn-cpp/PublisherPublicKeyDigest.hpp
+++ b/ndn-cpp/PublisherPublicKeyDigest.hpp
@@ -38,7 +38,7 @@
    * Clear this PublisherPublicKeyDigest, and copy from the ndn_PublisherPublicKeyDigest struct.
    * @param excludeStruct a C ndn_Exclude struct
    */
-  void set(struct ndn_PublisherPublicKeyDigest &publisherPublicKeyDigestStruct) 
+  void set(const struct ndn_PublisherPublicKeyDigest &publisherPublicKeyDigestStruct) 
   {
   	publisherPublicKeyDigest_.clear();
     if (publisherPublicKeyDigestStruct.publisherPublicKeyDigest)