diff --git a/swh/storage/buffer.py b/swh/storage/buffer.py
--- a/swh/storage/buffer.py
+++ b/swh/storage/buffer.py
@@ -140,7 +140,8 @@
             for batch in batches:
                 add_fn = getattr(self.storage, "%s_add" % object_type)
                 stats = add_fn(list(batch))
-                summary = {k: v + summary.get(k, 0) for k, v in stats.items()}
+                for k, v in stats.items():
+                    summary[k] = v + summary.get(k, 0)
 
         self.clear_buffers(object_types)
 
diff --git a/swh/storage/tests/test_buffer.py b/swh/storage/tests/test_buffer.py
--- a/swh/storage/tests/test_buffer.py
+++ b/swh/storage/tests/test_buffer.py
@@ -523,3 +523,30 @@
             methods_called,
         )
         prev = cur
+
+
+def test_buffer_flush_stats(sample_data) -> None:
+    storage = get_storage_with_buffer_config()
+
+    s = storage.content_add(sample_data.contents)
+    assert s == {}
+    s = storage.skipped_content_add(sample_data.skipped_contents)
+    assert s == {}
+    s = storage.directory_add(sample_data.directories)
+    assert s == {}
+    s = storage.revision_add(sample_data.revisions)
+    assert s == {}
+    s = storage.release_add(sample_data.releases)
+    assert s == {}
+    s = storage.snapshot_add(sample_data.snapshots)
+    assert s == {}
+
+    # Flush all the things
+    s = storage.flush()
+    assert s["content:add"] > 0
+    assert s["content:add:bytes"] > 0
+    assert s["skipped_content:add"] > 0
+    assert s["directory:add"] > 0
+    assert s["revision:add"] > 0
+    assert s["release:add"] > 0
+    assert s["snapshot:add"] > 0