diff --git a/m-tuple.h b/m-tuple.h
index 6ce32958..6d15b03c 100644
--- a/m-tuple.h
+++ b/m-tuple.h
@@ -212,7 +212,7 @@ namespace m_tuple {
   M_MAP2(TUPLE_DEFINE_SETTER_PROTO, name, __VA_ARGS__)
 #define TUPLE_DEFINE_SETTER_PROTO(name, a)                             \
   static inline void M_C3(name, _set_, TUPLE_GET_FIELD a)              \
-       (M_C(name,_t) my, TUPLE_GET_TYPE a TUPLE_GET_FIELD a) {         \
+       (M_C(name,_t) my, const TUPLE_GET_TYPE a TUPLE_GET_FIELD a) {   \
     TUPLE_GET_SET a (my ->TUPLE_GET_FIELD a, TUPLE_GET_FIELD a);}