diff --git a/types/tagged.hpp b/types/tagged.hpp
index 03b4c590..2a57528d 100644
--- a/types/tagged.hpp
+++ b/types/tagged.hpp
@@ -52,6 +52,7 @@ namespace cruft {
 
         //---------------------------------------------------------------------
         template <typename InitialT>
+        requires (cruft::concepts::same_as<std::remove_cvref_t<InitialT>, ValueT> || ...)
         tagged (InitialT &&initial)
         {
             set<InitialT> (std::forward<InitialT> (initial));