diff --git a/borgmatic/borg/diff.py b/borgmatic/borg/diff.py
index 245f6b56..3de9f9c0 100644
--- a/borgmatic/borg/diff.py
+++ b/borgmatic/borg/diff.py
@@ -75,8 +75,7 @@ def diff(
             else flags.make_repository_archive_flags(repository, archive, local_borg_version)
         )
         + (('--same-chunker-params',) if diff_arguments.same_chunker_params else ())
-        + (('--sort-by', diff_arguments.sort_by) if diff_arguments.sort_by else ())
-        + (tuple(diff_arguments.sort_keys) if diff_arguments.sort_keys else ())
+        + (('--sort-by', ','.join(diff_arguments.sort_keys)) if diff_arguments.sort_keys else ())
         + (('--content-only',) if diff_arguments.content_only else ())
         + (diff_arguments.second_archive, )
     )