diff --git a/include/boost/concept_archetype.hpp b/include/boost/concept_archetype.hpp
index 3343165..9981ebf 100644
--- a/include/boost/concept_archetype.hpp
+++ b/include/boost/concept_archetype.hpp
@@ -418,7 +418,7 @@ namespace boost {
     typedef std::input_iterator_tag iterator_category;
     typedef T value_type;
     struct reference {
-      operator value_type() { return static_object<T>::get(); }
+      operator value_type() const { return static_object<T>::get(); }
     };
     typedef const T* pointer;
     typedef std::ptrdiff_t difference_type;