diff --git a/src/_zkapauthorizer/controller.py b/src/_zkapauthorizer/controller.py
index 8823406016f10b27c8bea101b426d6b25034be20..f58192bbe09d3c9e8d86e594aa83c8741f474ba4 100644
--- a/src/_zkapauthorizer/controller.py
+++ b/src/_zkapauthorizer/controller.py
@@ -52,6 +52,10 @@ from .model import Voucher
 RETRY_INTERVAL = timedelta(milliseconds=1000)
 
 
+# It would be nice to have frozen exception types but Failure.cleanFailure
+# interacts poorly with these.
+# https://twistedmatrix.com/trac/ticket/9641
+# https://twistedmatrix.com/trac/ticket/9771
 @attr.s(auto_attribs=True)
 class UnexpectedResponse(Exception):
     """