diff --git a/include/msgpack/object_fwd.hpp b/include/msgpack/object_fwd.hpp
index e7a04c78..a8ae4422 100644
--- a/include/msgpack/object_fwd.hpp
+++ b/include/msgpack/object_fwd.hpp
@@ -137,6 +137,11 @@ struct object_kv {
     object val;
 };
 
+namespace detail {
+template <typename Stream, typename T>
+struct packer_serializer;
+} // namespace detail
+
 object const& operator>> (object const& o, object& v);
 
 template <typename T>