diff --git a/src/core/include/mp-units/framework/system_reference.h b/src/core/include/mp-units/framework/system_reference.h
index bfc4b81a..b4c202ec 100644
--- a/src/core/include/mp-units/framework/system_reference.h
+++ b/src/core/include/mp-units/framework/system_reference.h
@@ -68,9 +68,9 @@ struct system_reference {
   template<Unit U>
     requires(convertible(coherent_unit, U{}))
 #if MP_UNITS_COMP_MSVC
-  [[nodiscard]] constexpr decltype(reference<MP_UNITS_REMOVE_CONST(decltype(quantity_spec)), U>{}) operator[](U) const
+  [[nodiscard]] constexpr decltype(reference<MP_UNITS_REMOVE_CONST(decltype(Q)), U>{}) operator[](U) const
 #else
-  [[nodiscard]] constexpr reference<MP_UNITS_REMOVE_CONST(decltype(quantity_spec)), U> operator[](U) const
+  [[nodiscard]] constexpr reference<MP_UNITS_REMOVE_CONST(decltype(Q)), U> operator[](U) const
 #endif
   {
     return {};