diff --git a/plumbum/commands/modifiers.py b/plumbum/commands/modifiers.py
index f04d4faa..654b11b2 100644
--- a/plumbum/commands/modifiers.py
+++ b/plumbum/commands/modifiers.py
@@ -59,7 +59,7 @@ def returncode(self):
 
 
 class ExecutionModifier(object):
-    __slots__ = ()
+    __slots__ = ("__weakref__")
 
     def __repr__(self):
         """Automatically creates a representation for given subclass with slots.