diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 76bbb909..50c777c8 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -261,7 +261,7 @@ namespace detail {
 using utc_clock = std::chrono::utc_clock;
 #else
 struct utc_clock {
-  void to_sys();
+  template <typename T> void to_sys(T);
 };
 #endif