diff --git a/combo/apps/lingo/models.py b/combo/apps/lingo/models.py
index 697e5e75..fd7e6da4 100644
--- a/combo/apps/lingo/models.py
+++ b/combo/apps/lingo/models.py
@@ -122,7 +122,7 @@ class Regie(models.Model):
 
     def get_invoices(self, user, history=False):
         if not self.is_remote():
-            return self.basketitem_set.filter(payment_date__isnull=boot(not history), user=user)
+            return self.basketitem_set.filter(payment_date__isnull=bool(not history), user=user)
         if user:
             url = self.webservice_url + '/invoices/'
             if history: