diff --git a/swh/lister/gnu/lister.py b/swh/lister/gnu/lister.py
index 7d976ee..3d35829 100644
--- a/swh/lister/gnu/lister.py
+++ b/swh/lister/gnu/lister.py
@@ -1,68 +1,73 @@
 # Copyright (C) 2019-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import logging
-from typing import Any, Iterator, Mapping
+from typing import Any, Iterator, Mapping, Optional
 
 import iso8601
 
 from swh.scheduler.interface import SchedulerInterface
 from swh.scheduler.model import ListedOrigin
 
 from ..pattern import CredentialsType, StatelessLister
 from .tree import GNUTree
 
 logger = logging.getLogger(__name__)
 
 GNUPageType = Mapping[str, Any]
 
 
 class GNULister(StatelessLister[GNUPageType]):
     """
     List all GNU projects and associated artifacts.
     """
 
     LISTER_NAME = "GNU"
     GNU_FTP_URL = "https://ftp.gnu.org"
 
     def __init__(
         self, scheduler: SchedulerInterface, credentials: CredentialsType = None,
     ):
         super().__init__(
             scheduler=scheduler,
             url=self.GNU_FTP_URL,
             instance="GNU",
             credentials=credentials,
         )
-        self.gnu_tree = GNUTree(f"{self.url}/tree.json.gz")
+        # no side-effect calls in constructor, if extra state is needed, as preconized
+        # by the pattern docstring, this must happen in the get_pages method.
+        self.gnu_tree: Optional[GNUTree] = None
 
     def get_pages(self) -> Iterator[GNUPageType]:
         """
         Yield a single page listing all GNU projects.
         """
+        # first fetch the manifest to parse
+        self.gnu_tree = GNUTree(f"{self.url}/tree.json.gz")
         yield self.gnu_tree.projects
 
     def get_origins_from_page(self, page: GNUPageType) -> Iterator[ListedOrigin]:
         """
         Iterate on all GNU projects and yield ListedOrigin instances.
         """
         assert self.lister_obj.id is not None
+        assert self.gnu_tree is not None
 
         artifacts = self.gnu_tree.artifacts
 
         for project_name, project_info in page.items():
 
             origin_url = project_info["url"]
             last_update = iso8601.parse_date(project_info["time_modified"])
 
             logger.debug("Found origin %s last updated on %s", origin_url, last_update)
 
             yield ListedOrigin(
                 lister_id=self.lister_obj.id,
                 url=origin_url,
                 visit_type="tar",
                 last_update=last_update,
                 extra_loader_arguments={"artifacts": artifacts[project_name]},
             )