diff --git a/src/restic_compose_backup/restic.py b/src/restic_compose_backup/restic.py
index 6bdee08..35753b5 100644
--- a/src/restic_compose_backup/restic.py
+++ b/src/restic_compose_backup/restic.py
@@ -80,7 +80,7 @@ def forget(repository: str, keeplast: str, hourly: str, daily: str, weekly: str,
         'forget',
         '--group-by',
         'paths,tags',
-        '-keep-last',
+        '--keep-last',
         keeplast,
         '--keep-hourly',
         hourly,