diff --git a/Doc/library/copy.rst b/Doc/library/copy.rst
index a8e8bfb..176e01d 100644
--- a/Doc/library/copy.rst
+++ b/Doc/library/copy.rst
@@ -86,6 +86,7 @@
 argument, the ``memo`` dictionary.  If the :meth:`__deepcopy__` implementation needs
 to make a deep copy of a component, it should call the :func:`deepcopy` function
 with the component as first argument and the memo dictionary as second argument.
+The memo dictionary should be treated as an opaque object.
 
 
 .. seealso::