diff --git a/keystoneclient/shell.py b/keystoneclient/shell.py
index 126e08292..e61bf2e9d 100644
--- a/keystoneclient/shell.py
+++ b/keystoneclient/shell.py
@@ -131,7 +131,7 @@ class OpenStackIdentityShell(object):
 
         parser.add_argument('--os-cacert',
                             metavar='<ca-certificate>',
-                            default=env('OS_CA_CERT'),
+                            default=env('OS_CA_CERT', default=None),
                             help='Defaults to env[OS_CACERT]')
         parser.add_argument('--os_cacert',
                             help=argparse.SUPPRESS)