diff --git a/include/raii/util.hpp b/include/raii/util.hpp
index 6f91145..64d97da 100644
--- a/include/raii/util.hpp
+++ b/include/raii/util.hpp
@@ -44,7 +44,7 @@ namespace raii{
 			return len;
 		}
 	}
-	template<class T, typename std::enable_if<rexy::detail::is_string<T>::value && !rexy::detail::is_concrete_string<T>::value,void>::type* = nullptr>
+	template<class T, typename std::enable_if<rexy::is_string<T>::value && !rexy::is_concrete_string<T>::value,void>::type* = nullptr>
 	rexy::string json_escape(T&& t){
 		auto tup = t.get();
 		size_t len = detail::_calc_escaped_len_all(tup);