diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index e6e3013e..fb32cc99 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -452,9 +452,11 @@ template<detail::QuantitySpecWithNoSpecifiers auto Q>
   requires(get_kind(Q) == Q)
 #ifdef __cpp_explicit_this_parameter
 struct kind_of_<Q> : Q {
+  static constexpr auto _quantity_spec_ = Q;
 };
 #else
 struct kind_of_<Q> : quantity_spec<kind_of_<Q>, Q> {
+  static constexpr auto _quantity_spec_ = Q;
 };
 #endif