diff --git a/vdirsyncer/cli.py b/vdirsyncer/cli.py
index de3dcfc..e1f9843 100644
--- a/vdirsyncer/cli.py
+++ b/vdirsyncer/cli.py
@@ -35,8 +35,8 @@ except ImportError:
 
 cli_logger = log.get(__name__)
 
-GENERAL_ALL = set(['status_path', 'passwordeval'])
-GENERAL_REQUIRED = set(['status_path'])
+GENERAL_ALL = frozenset(['status_path', 'passwordeval'])
+GENERAL_REQUIRED = frozenset(['status_path'])
 
 
 class CliError(RuntimeError):