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
@@ -189,6 +189,10 @@
                 seen_revisions.add(branch.target)
                 revision = storage.revision_get([branch.target])[0]
 
+                if revision is None:
+                    # https://forge.softwareheritage.org/T997
+                    continue
+
                 # Check it's DSC (we only support those for now)
                 assert (
                     revision.type == RevisionType.DSC
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
@@ -214,6 +214,27 @@
             with patch("snapshot_get_all_branches", return_value=snapshot):
                 assert debian_origins_from_row(revision_row, storage) == []
 
+    snapshot = attr.evolve(
+        snapshot,
+        branches={
+            b"foo": SnapshotBranch(target_type=TargetType.REVISION, target=b"21" * 10,)
+        },
+    )
+    storage.revision_get.return_value = [None]
+
+    # branch points to unknown revision
+    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 == [
+        call.revision_get([b"21212121212121212121"]),
+        call.revision_get([b"21212121212121212121"]),
+        call.revision_get([b"21212121212121212121"]),
+    ]
+    storage.reset_mock()
+
     revision = Revision(
         id=b"21" * 10,
         message=b"foo",
@@ -245,7 +266,11 @@
             with patch("snapshot_get_all_branches", return_value=snapshot):
                 assert debian_origins_from_row(revision_row, storage) == []
 
-    assert storage.method_calls == []
+    assert storage.method_calls == [
+        call.revision_get([b"21212121212121212121"]),
+        call.revision_get([b"21212121212121212121"]),
+        call.revision_get([b"21212121212121212121"]),
+    ]
 
 
 def test_debian_origins_from_row__check_revisions():