diff --git a/src/security/certificate/certificate-extension.cpp b/src/security/certificate/certificate-extension.cpp
index b2c1bad..f081f2a 100644
--- a/src/security/certificate/certificate-extension.cpp
+++ b/src/security/certificate/certificate-extension.cpp
@@ -11,18 +11,17 @@
 #include <ndn-cpp/security/certificate/certificate-extension.hpp>
 
 using namespace std;
-using namespace ndn::ptr_lib;
 
 namespace ndn {
 
-shared_ptr<der::DerNode> 
+ptr_lib::shared_ptr<der::DerNode> 
 CertificateExtension::toDer()
 {
-  shared_ptr<der::DerSequence> root(new der::DerSequence);
+  ptr_lib::shared_ptr<der::DerSequence> root(new der::DerSequence);
     
-  shared_ptr<der::DerOid> extensionId(new der::DerOid(extensionId_));
-  shared_ptr<der::DerBool> isCritical(new der::DerBool(isCritical_));
-  shared_ptr<der::DerOctetString> extensionValue(new der::DerOctetString(*extensionValue_));
+  ptr_lib::shared_ptr<der::DerOid> extensionId(new der::DerOid(extensionId_));
+  ptr_lib::shared_ptr<der::DerBool> isCritical(new der::DerBool(isCritical_));
+  ptr_lib::shared_ptr<der::DerOctetString> extensionValue(new der::DerOctetString(*extensionValue_));
 
   root->addChild(extensionId);
   root->addChild(isCritical);