diff --git a/src/detail/probe-encoder.hpp b/src/detail/probe-encoder.hpp
index bd15463..815a840 100644
--- a/src/detail/probe-encoder.hpp
+++ b/src/detail/probe-encoder.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2017-2022, Regents of the University of California.
+ * Copyright (c) 2017-2024, Regents of the University of California.
  *
  * This file is part of ndncert, a certificate management system based on NDN.
  *
@@ -23,6 +23,8 @@
 
 #include "detail/ndncert-common.hpp"
 
+#include <optional>
+
 namespace ndncert::probetlv {
 
 // For Client use