diff --git a/tools/nfdc/canonizer.cpp b/tools/nfdc/canonizer.cpp
index 3e526cd..a44a2bf 100644
--- a/tools/nfdc/canonizer.cpp
+++ b/tools/nfdc/canonizer.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2020,  Regents of the University of California,
+ * Copyright (c) 2014-2022,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -29,10 +29,10 @@
 namespace tools {
 namespace nfdc {
 
-std::pair<optional<FaceUri>, std::string>
+std::pair<std::optional<FaceUri>, std::string>
 canonize(ExecuteContext& ctx, const FaceUri& uri)
 {
-  optional<FaceUri> result;
+  std::optional<FaceUri> result;
   std::string error;
   uri.canonize(
     [&result] (const FaceUri& canonicalUri) { result = canonicalUri; },