diff --git a/rinoh/backend/pdf/cos.py b/rinoh/backend/pdf/cos.py
index <HASH>..<HASH> 100644
--- a/rinoh/backend/pdf/cos.py
+++ b/rinoh/backend/pdf/cos.py
@@ -70,9 +70,7 @@ class Object(object):
             document.register(self)
 
 
-NoneType = type(None)
-
-class Null(Object, NoneType):
+class Null(Object):
     def __init__(self, indirect=False):
         super().__init__(indirect)
Don't inherit from NoneType

bool(Null()) == True anyway, so there're no reason to do this.