diff --git a/uncloud_pay/views.py b/uncloud_pay/views.py
index 1144b49..9ca3bcf 100644
--- a/uncloud_pay/views.py
+++ b/uncloud_pay/views.py
@@ -197,6 +197,9 @@ class BillViewSet(viewsets.ReadOnlyModelViewSet):
 
     @action(detail=True, methods=['get'])
     def download(self, *args, **kwargs):
+        """
+        Allow to download
+        """
         bill = self.get_object()
         output_file = NamedTemporaryFile()
         bill_html = render_to_string("bill.html.j2", {'bill': bill})