diff --git a/include/fmt/std.h b/include/fmt/std.h
index 1e69b8b2..ae485344 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -211,7 +211,7 @@ template <typename Char>
 struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};
 FMT_END_NAMESPACE
 
-#ifdef __cpp_lib_optional
+#ifdef __cpp_lib_optional_
 FMT_BEGIN_NAMESPACE
 FMT_EXPORT
 template <typename T, typename Char>
diff --git a/test/std-test.cc b/test/std-test.cc
index bcc7bd5f..24981d87 100644
--- a/test/std-test.cc
+++ b/test/std-test.cc
@@ -56,7 +56,7 @@ TEST(ranges_std_test, format_quote_path) {
       std::vector<std::filesystem::path>{"path1/file1.txt", "path2/file2.txt"};
   EXPECT_EQ(fmt::format("{}", vec),
             "[\"path1/file1.txt\", \"path2/file2.txt\"]");
-#  ifdef __cpp_lib_optional
+#  ifdef __cpp_lib_optional_
   auto o = std::optional<std::filesystem::path>("path/file.txt");
   EXPECT_EQ(fmt::format("{}", o), "optional(\"path/file.txt\")");
   EXPECT_EQ(fmt::format("{:?}", o), "optional(\"path/file.txt\")");
@@ -109,7 +109,7 @@ TEST(std_test, source_location) {
 #endif
 
 TEST(std_test, optional) {
-#ifdef __cpp_lib_optional
+#ifdef __cpp_lib_optional_
   EXPECT_EQ(fmt::format("{}", std::optional<int>{}), "none");
   EXPECT_EQ(fmt::format("{}", std::pair{1, "second"}), "(1, \"second\")");
   EXPECT_EQ(fmt::format("{}", std::vector{std::optional{1}, std::optional{2},
@@ -184,7 +184,7 @@ class my_class {
 };
 }  // namespace my_nso
 TEST(std_test, optional_format_as) {
-#ifdef __cpp_lib_optional
+#ifdef __cpp_lib_optional_
   EXPECT_EQ(fmt::format("{}", std::optional<my_nso::my_number>{}), "none");
   EXPECT_EQ(fmt::format("{}", std::optional{my_nso::my_number::one}),
             "optional(\"first\")");
diff --git a/test/xchar-test.cc b/test/xchar-test.cc
index 312e632b..6210e244 100644
--- a/test/xchar-test.cc
+++ b/test/xchar-test.cc
@@ -590,7 +590,7 @@ TEST(std_test_xchar, complex) {
 }
 
 TEST(std_test_xchar, optional) {
-#  ifdef __cpp_lib_optional
+#  ifdef __cpp_lib_optional_
   EXPECT_EQ(fmt::format(L"{}", std::optional{L'C'}), L"optional(\'C\')");
   EXPECT_EQ(fmt::format(L"{}", std::optional{std::wstring{L"wide string"}}),
             L"optional(\"wide string\")");