diff --git a/tests/unit/security/validator-fixture.hpp b/tests/unit/security/validator-fixture.hpp
index d8d357f..7e1e0b5 100644
--- a/tests/unit/security/validator-fixture.hpp
+++ b/tests/unit/security/validator-fixture.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2022 Regents of the University of California.
+ * Copyright (c) 2013-2023 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -158,7 +158,7 @@
 
 private:
   void
-  verifyOriginalPacket(const optional<Certificate>&) override
+  verifyOriginalPacket(const std::optional<Certificate>&) override
   {
     // do nothing
   }