diff --git a/src/security/certificate-extension.hpp b/src/security/certificate-extension.hpp
index a1aa5b6..83653ee 100644
--- a/src/security/certificate-extension.hpp
+++ b/src/security/certificate-extension.hpp
@@ -13,7 +13,9 @@
 #include "../encoding/buffer.hpp"
 #include "../encoding/oid.hpp"
 
-namespace CryptoPP { class BufferedTransformation; }
+namespace CryptoPP {
+class BufferedTransformation;
+}
 
 namespace ndn {