diff --git a/letsencrypt/plugins/standalone.py b/letsencrypt/plugins/standalone.py
index b76095f89..17e99073f 100644
--- a/letsencrypt/plugins/standalone.py
+++ b/letsencrypt/plugins/standalone.py
@@ -176,6 +176,7 @@ class Authenticator(common.Plugin):
 
     @property
     def supported_challenges(self):
+        """Challenges supported by this plugin."""
         return set(challenges.Challenge.TYPES[name] for name in
                    self.conf("supported-challenges").split(","))