diff --git a/src/borg/cache.py b/src/borg/cache.py
index 05a543b30..7dddc93aa 100644
--- a/src/borg/cache.py
+++ b/src/borg/cache.py
@@ -601,7 +601,7 @@ def write_archive_index(archive_id, chunk_idx):
             fn_tmp = mkpath(archive_id, suffix='.tmp')
             try:
                 with DetachedIntegrityCheckedFile(path=fn_tmp, write=True,
-                                                  filename=bin_to_hex(archive_id)) as fd:
+                                                  filename=bin_to_hex(archive_id) + '.compact') as fd:
                     chunk_idx.write(fd)
             except Exception:
                 os.unlink(fn_tmp)