diff --git a/swh/provenance/swhgraph/archive.py b/swh/provenance/swhgraph/archive.py
index 2c85515..93cd3c8 100644
--- a/swh/provenance/swhgraph/archive.py
+++ b/swh/provenance/swhgraph/archive.py
@@ -1,50 +1,51 @@
 # Copyright (C) 2022  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
 
 from typing import Any, Dict, Iterable, Tuple
 
 from swh.core.statsd import statsd
 from swh.model.model import Sha1Git
 from swh.model.swhids import CoreSWHID, ObjectType
 from swh.storage.interface import StorageInterface
 
 ARCHIVE_DURATION_METRIC = "swh_provenance_archive_graph_duration_seconds"
 
 
 class ArchiveGraph:
     def __init__(self, graph, storage: StorageInterface) -> None:
         self.graph = graph
         self.storage = storage  # required by ArchiveInterface
 
     @statsd.timed(metric=ARCHIVE_DURATION_METRIC, tags={"method": "directory_ls"})
     def directory_ls(self, id: Sha1Git, minsize: int = 0) -> Iterable[Dict[str, Any]]:
         raise NotImplementedError
 
     @statsd.timed(
         metric=ARCHIVE_DURATION_METRIC,
         tags={"method": "revision_get_some_outbound_edges"},
     )
     def revision_get_some_outbound_edges(
         self, revision_id: Sha1Git
     ) -> Iterable[Tuple[Sha1Git, Sha1Git]]:
         src = CoreSWHID(object_type=ObjectType.REVISION, object_id=revision_id)
         request = self.graph.visit_edges(str(src), edges="rev:rev")
 
-        for rev_swhid, parent_rev_swhid in request:
-            yield (
-                CoreSWHID.from_string(rev_swhid).object_id,
-                CoreSWHID.from_string(parent_rev_swhid).object_id,
-            )
+        for edge in request:
+            if edge:
+                yield (
+                    CoreSWHID.from_string(edge[0]).object_id,
+                    CoreSWHID.from_string(edge[1]).object_id,
+                )
 
     @statsd.timed(metric=ARCHIVE_DURATION_METRIC, tags={"method": "snapshot_get_heads"})
     def snapshot_get_heads(self, id: Sha1Git) -> Iterable[Sha1Git]:
         src = CoreSWHID(object_type=ObjectType.SNAPSHOT, object_id=id)
         request = self.graph.visit_nodes(
             str(src), edges="snp:rev,snp:rel,rel:rev", return_types="rev"
         )
 
         yield from (
             CoreSWHID.from_string(swhid).object_id for swhid in request if swhid
         )