diff --git a/strongdef.hpp b/strongdef.hpp
index 78bdd231..9de4beaa 100644
--- a/strongdef.hpp
+++ b/strongdef.hpp
@@ -25,6 +25,10 @@ namespace cruft::strongdef {
         using value_type = T;
         using tag_type   = TagT;
 
+        /// We detect underlying_type in a few situations where we emulate
+        /// 'enum-like' structures. This lets us opt in to this behaviour.
+        using underlying_type = T;
+
         // TODO: ideally we'd default the constructor, but templated
         // constructors can't be defaulted? So we just stub one out.
         constexpr explicit index () { ; }
@@ -144,9 +148,8 @@ namespace cruft::types {
     template <typename TagT, typename ValueT>
     struct category_traits<
         ::cruft::strongdef::index<TagT,ValueT>
-    > :
-        public category_traits<ValueT>
-    {};
+    > : public std::integral_constant<category,category::ENUM>
+    { ; };
 
 
     template <typename TagT, typename ValueT>