diff --git a/swh/storage/postgresql/storage.py b/swh/storage/postgresql/storage.py
--- a/swh/storage/postgresql/storage.py
+++ b/swh/storage/postgresql/storage.py
@@ -914,9 +914,13 @@
         db: Db,
         cur=None,
     ) -> Optional[PartialBranches]:
+
         if snapshot_id == EMPTY_SNAPSHOT_ID:
             return PartialBranches(id=snapshot_id, branches={}, next_branch=None,)
 
+        if list(self.snapshot_missing([snapshot_id])):
+            return None
+
         branches = {}
         next_branch = None
 
diff --git a/swh/storage/tests/storage_tests.py b/swh/storage/tests/storage_tests.py
--- a/swh/storage/tests/storage_tests.py
+++ b/swh/storage/tests/storage_tests.py
@@ -22,6 +22,7 @@
 from swh.model import from_disk
 from swh.model.hashutil import DEFAULT_ALGORITHMS, hash_to_bytes
 from swh.model.hypothesis_strategies import objects
+from swh.model.hypothesis_strategies import snapshots as unknown_snapshot
 from swh.model.identifiers import CoreSWHID, ObjectType
 from swh.model.model import (
     Content,
@@ -3656,6 +3657,12 @@
         assert partial_branches is not None
         assert partial_branches["branches"] == {}
 
+    @settings(suppress_health_check=function_scoped_fixture_check,)
+    @given(unknown_snapshot(min_size=1))
+    def test_snapshot_get_unknown_snapshot(self, swh_storage, unknown_snapshot):
+        assert swh_storage.snapshot_get(unknown_snapshot.id) is None
+        assert swh_storage.snapshot_get_branches(unknown_snapshot.id) is None
+
     def test_snapshot_add_get(self, swh_storage, sample_data):
         snapshot = sample_data.snapshot
         origin = sample_data.origin