From a90ad5d6fde4b39417f63a034d566dd7f09b0a55 Mon Sep 17 00:00:00 2001
From: Jiami Lin <carmel0@me.com>
Date: Tue, 2 Jun 2026 09:16:49 +0800
Subject: [PATCH] Fix formatting std::tm with null tm_zone (#4790)

---
 include/fmt/chrono.h | 1 +
 test/chrono-test.cc  | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 517f9576..e4bc1ce4 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -1192,6 +1192,7 @@ class tm_writer {
 
   template <typename T, FMT_ENABLE_IF(has_tm_zone<T>::value)>
   void format_tz_name(const T& tm) {
+    if (!tm.tm_zone) FMT_THROW(format_error("no timezone"));
     out_ = write_tm_str<Char>(out_, tm.tm_zone, loc_);
   }
   template <typename T, FMT_ENABLE_IF(!has_tm_zone<T>::value)>
diff --git a/test/chrono-test.cc b/test/chrono-test.cc
index 7b4bf402..a9153598 100644
--- a/test/chrono-test.cc
+++ b/test/chrono-test.cc
@@ -358,6 +358,9 @@ TEST(chrono_test, tm) {
   char tz[] = "EET";
   if (fmt::detail::set_tm_zone(time, tz)) {
     EXPECT_EQ(fmt::format(fmt::runtime("{:%Z}"), time), "EET");
+    fmt::detail::set_tm_zone(time, nullptr);
+    EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%Z}"), time),
+                     fmt::format_error, "no timezone");
   } else {
     EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%Z}"), time),
                      fmt::format_error, "no timezone");