diff --git a/rdflib/namespace/__init__.py b/rdflib/namespace/__init__.py
index 06647a867..eb8e2eeed 100644
--- a/rdflib/namespace/__init__.py
+++ b/rdflib/namespace/__init__.py
@@ -226,6 +226,7 @@ def __repr__(self) -> str:
 # considered part of __dir__ results. These should be all annotations on
 # `DefinedNamespaceMeta`.
 _DFNS_RESERVED_ATTRS: Set[str] = {
+    "__slots__",
     "_NS",
     "_warn",
     "_fail",