diff --git a/src/os.cc b/src/os.cc
index 4d7c299b..f21d062c 100644
--- a/src/os.cc
+++ b/src/os.cc
@@ -59,6 +59,8 @@
 #  endif
 #endif
 
+FMT_BEGIN_NAMESPACE
+
 namespace {
 #ifdef _WIN32
 // Return type of read and write functions.
@@ -77,11 +79,11 @@ inline auto convert_rwcount(size_t count) -> size_t { return count; }
 #endif
 }  // namespace
 
-FMT_BEGIN_NAMESPACE
-
 #ifdef _WIN32
 namespace detail {
 
+namespace {
+
 class system_message {
   system_message(const system_message&) = delete;
   void operator=(const system_message&) = delete;
@@ -129,6 +131,8 @@ class utf8_system_category final : public std::error_category {
   }
 };
 
+}  // namespace
+
 }  // namespace detail
 
 FMT_API const std::error_category& system_category() noexcept {