diff --git a/swh/storage/algos/diff.py b/swh/storage/algos/diff.py
--- a/swh/storage/algos/diff.py
+++ b/swh/storage/algos/diff.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2018  The Software Heritage developers
+# Copyright (C) 2018-2020  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
@@ -14,20 +14,27 @@
 
 import collections
 
+from typing import Any, Dict
+
 from swh.model.hashutil import hash_to_bytes
 from swh.model.identifiers import directory_identifier
 
+from swh.storage.interface import StorageInterface
+
 from .dir_iterators import DirectoryIterator, DoubleDirectoryIterator, Remaining
 
+
 # get the hash identifier for an empty directory
 _empty_dir_hash = hash_to_bytes(directory_identifier({"entries": []}))
 
 
-def _get_rev(storage, rev_id):
+def _get_rev(storage: StorageInterface, rev_id: bytes) -> Dict[str, Any]:
     """
     Return revision data from swh storage.
     """
-    return list(storage.revision_get([rev_id]))[0]
+    revision = storage.revision_get([rev_id])[0]
+    assert revision is not None
+    return revision.to_dict()
 
 
 class _RevisionChangesList(object):
diff --git a/swh/storage/tests/algos/test_diff.py b/swh/storage/tests/algos/test_diff.py
--- a/swh/storage/tests/algos/test_diff.py
+++ b/swh/storage/tests/algos/test_diff.py
@@ -1,11 +1,13 @@
-# Copyright (C) 2018  The Software Heritage developers
+# Copyright (C) 2018-2020  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
 
 # flake8: noqa
 
+import pytest
 import unittest
+
 from unittest.mock import patch
 
 from swh.model.hashutil import hash_to_bytes
@@ -15,6 +17,19 @@
 from .test_dir_iterator import DirectoryModel
 
 
+def test__get_rev(swh_storage, sample_data):
+    revision = sample_data.revision
+
+    # does not exist then raises
+    with pytest.raises(AssertionError):
+        diff._get_rev(swh_storage, revision.id)
+
+    # otherwise, we retrieve its dict representation
+    swh_storage.revision_add([revision])
+    actual_revision = diff._get_rev(swh_storage, revision.id)
+    assert actual_revision == revision.to_dict()
+
+
 @patch("swh.storage.algos.diff._get_rev")
 @patch("swh.storage.algos.dir_iterators._get_dir")
 class TestDiffRevisions(unittest.TestCase):