diff --git a/src/security/certificate-cache-ttl.hpp b/src/security/certificate-cache-ttl.hpp
index 31f8255..650a9e7 100644
--- a/src/security/certificate-cache-ttl.hpp
+++ b/src/security/certificate-cache-ttl.hpp
@@ -33,6 +33,7 @@
 class CertificateCacheTtl : public CertificateCache
 {
 public:
+  explicit
   CertificateCacheTtl(boost::asio::io_service& io,
                       const time::seconds& defaultTtl = time::seconds(3600))
     : m_defaultTtl(defaultTtl)