diff --git a/dunning.py b/dunning.py
index 5c842cfaf3..b1071af7bd 100644
--- a/dunning.py
+++ b/dunning.py
@@ -95,6 +95,8 @@ class Letter(CompanyReport, metaclass=PoolMeta):
     def get_party_letter():
 
         class PartyLetter(object, metaclass=PoolMeta):
+            __slots__ = ('dunnings', 'payments')
+
             def __init__(self, dunnings, payments):
                 self.dunnings = dunnings
                 self.payments = payments