diff --git a/swh/loader/git/loader.py b/swh/loader/git/loader.py
--- a/swh/loader/git/loader.py
+++ b/swh/loader/git/loader.py
@@ -326,7 +326,7 @@
         if self.dumb:
             self.dumb_fetcher = dumb.GitObjectsFetcher(self.origin.url, base_repo)
             self.dumb_fetcher.fetch_object_ids()
-            self.remote_refs = utils.filter_refs(self.dumb_fetcher.refs)  # type: ignore
+            self.remote_refs = utils.filter_refs(self.dumb_fetcher.refs)
             self.symbolic_refs = self.dumb_fetcher.head
         else:
             self.pack_buffer = fetch_info.pack_buffer
diff --git a/swh/loader/git/utils.py b/swh/loader/git/utils.py
--- a/swh/loader/git/utils.py
+++ b/swh/loader/git/utils.py
@@ -10,7 +10,7 @@
 import os
 import shutil
 import tempfile
-from typing import Dict, NewType, Optional
+from typing import Dict, Mapping, NewType, Optional, Union
 
 from swh.core import tarball
 from swh.model.model import SnapshotBranch
@@ -93,7 +93,7 @@
     return False
 
 
-def filter_refs(refs: Dict[bytes, bytes]) -> Dict[bytes, HexBytes]:
+def filter_refs(refs: Mapping[bytes, Union[bytes, HexBytes]]) -> Dict[bytes, HexBytes]:
     """Filter the refs dictionary using the policy set in `ignore_branch_name`"""
     return {
         name: HexBytes(target)