diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -162,7 +162,7 @@
 
         return b
 
-    def prepare_origin_visit(self, *args, **kwargs):
+    def prepare_origin_visit(self, *args, **kwargs) -> None:
         self.origin = Origin(url=self.origin_url)
         visit_date = self.visit_date
         if isinstance(visit_date, str):  # visit_date can be string or datetime