diff --git a/swh/loader/svn/tasks.py b/swh/loader/svn/tasks.py
--- a/swh/loader/svn/tasks.py
+++ b/swh/loader/svn/tasks.py
@@ -13,7 +13,10 @@
 
 
 def convert_to_datetime(date: Optional[str]) -> Optional[datetime]:
+    if date is None:
+        return None
     try:
+        assert isinstance(date, str)
         return iso8601.parse_date(date)
     except Exception:
         return None