diff --git a/btas/index_traits.h b/btas/index_traits.h
index c28bff13..f28fda23 100644
--- a/btas/index_traits.h
+++ b/btas/index_traits.h
@@ -26,7 +26,7 @@ class has_integral_value_type {
 /// test _Index conforms the TWG.Index concept
 /// check only value_type and operator[]
 template<typename ...>
-class is_index;
+class is_index : public std::false_type {};
 
 template<typename _Index>
 class is_index<_Index> {