diff --git a/include/raii/string_base.hpp b/include/raii/string_base.hpp
index 66f9251..eaaaffd 100644
--- a/include/raii/string_base.hpp
+++ b/include/raii/string_base.hpp
@@ -288,9 +288,10 @@ namespace raii{
 		Right m_r;
 
 	public:
-		constexpr string_cat_expr(Left l, Right r):
-			m_l(std::forward<Left>(l)),
-			m_r(std::forward<Right>(r)){}
+		template<class T, class U>
+		constexpr string_cat_expr(T&& l, U&& r):
+			m_l(std::forward<T>(l)),
+			m_r(std::forward<U>(r)){}
 		constexpr string_cat_expr(const string_cat_expr& s):
 			m_l(std::forward<Left>(s.m_l)),
 			m_r(std::forward<Right>(s.m_r)){}