diff --git a/swh/storage/migrate_extrinsic_metadata.py b/swh/storage/migrate_extrinsic_metadata.py
--- a/swh/storage/migrate_extrinsic_metadata.py
+++ b/swh/storage/migrate_extrinsic_metadata.py
@@ -176,6 +176,9 @@
             seen_snapshots.add(status.snapshot)
             snapshot = snapshot_get_all_branches(storage, status.snapshot)
             for (branch_name, branch) in snapshot.branches.items():
+                if branch is None:
+                    continue
+
                 # If it's the revision passed as argument, then it is indeed in the
                 # origin
                 if branch.target == revision_id:
diff --git a/swh/storage/tests/migrate_extrinsic_metadata/test_debian.py b/swh/storage/tests/migrate_extrinsic_metadata/test_debian.py
--- a/swh/storage/tests/migrate_extrinsic_metadata/test_debian.py
+++ b/swh/storage/tests/migrate_extrinsic_metadata/test_debian.py
@@ -216,6 +216,16 @@
             with patch("snapshot_get_all_branches", return_value=snapshot):
                 assert debian_origins_from_row(revision_row, storage) == []
 
+    snapshot = attr.evolve(snapshot, branches={b"foo": None,},)
+
+    # dangling branch
+    with patch("iter_origin_visits", return_value=[visit]):
+        with patch("iter_origin_visit_statuses", return_value=[status]):
+            with patch("snapshot_get_all_branches", return_value=snapshot):
+                assert debian_origins_from_row(revision_row, storage) == []
+
+    assert storage.method_calls == []
+
     snapshot = attr.evolve(
         snapshot,
         branches={