diff --git a/subprocess.hpp b/subprocess.hpp
index 24fa303..9029647 100755
--- a/subprocess.hpp
+++ b/subprocess.hpp
@@ -706,7 +706,7 @@ struct string_arg
 {
   string_arg(const char* arg): arg_value(arg) {}
   string_arg(std::string&& arg): arg_value(std::move(arg)) {}
-  string_arg(std::string arg): arg_value(std::move(arg)) {}
+  string_arg(const std::string& arg): arg_value(arg) {}
   std::string arg_value;
 };