diff --git a/ndindex/ndindex.py b/ndindex/ndindex.py
index 55c20db5..498b8882 100644
--- a/ndindex/ndindex.py
+++ b/ndindex/ndindex.py
@@ -210,6 +210,8 @@ def __hash__(self): # pragma: no cover
         return hash(self.args)
 
 class NDIndexBase:
+    __slots__ = ()
+
     def reduce(self, shape=None, *, negative_int=False):
         """
         Simplify an index given that it will be applied to an array of a given shape.