diff --git a/modoboa/admin/models/mixins.py b/modoboa/admin/models/mixins.py
index 82e0dcf6c..418c9ad12 100644
--- a/modoboa/admin/models/mixins.py
+++ b/modoboa/admin/models/mixins.py
@@ -22,6 +22,6 @@ class MessageLimitMixin:
     @property
     def sent_messages_in_percent(self):
         """Return number of sent messages as a percentage."""
-        if self.message_limit is None:
+        if not self.message_limit:
             return None
         return int(self.sent_messages / float(self.message_limit) * 100)