diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 5df03da74..3fbd75562 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -158,7 +158,7 @@ extern Verbosity verbosity; /* suppress msgs > this */
 #define vomit(args...) printMsg(lvlVomit, args)
 
 template<typename... Args>
-inline void warn(const std::string & fs, Args... args)
+inline void warn(const std::string & fs, const Args & ... args)
 {
     boost::format f(fs);
     nop{boost::io::detail::feed(f, args)...};
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 4bc91828b..4a6be28a2 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -67,7 +67,7 @@ inline std::string fmt(const FormatOrString & fs)
 }
 
 template<typename... Args>
-inline std::string fmt(const std::string & fs, Args... args)
+inline std::string fmt(const std::string & fs, const Args & ... args)
 {
     boost::format f(fs);
     f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);