diff --git a/src/security/certificate/certificate-subject-description.cpp b/src/security/certificate/certificate-subject-description.cpp
index 9aa511f..3571f7f 100644
--- a/src/security/certificate/certificate-subject-description.cpp
+++ b/src/security/certificate/certificate-subject-description.cpp
@@ -14,7 +14,7 @@
 namespace ndn {
 
 ptr_lib::shared_ptr<der::DerNode> 
-CertificateSubjectDescription::toDer()
+CertificateSubjectDescription::toDer() const
 {
   ptr_lib::shared_ptr<der::DerSequence> root(new der::DerSequence());