diff --git a/src/_zkapauthorizer/tests/test_storage_client.py b/src/_zkapauthorizer/tests/test_storage_client.py
index 6716ad7a44807c97c424f7be1a45e83d9f550ec3..61274b2c6869f89dedc70a9293e8052cd329bb0f 100644
--- a/src/_zkapauthorizer/tests/test_storage_client.py
+++ b/src/_zkapauthorizer/tests/test_storage_client.py
@@ -200,6 +200,8 @@ class CallWithPassesTests(TestCase):
         of passes, still of length ```num_passes``, but without the passes
         which were rejected on the first try.
         """
+        # Half of the passes are going to be rejected so make twice as many as
+        # the operation uses available.
         passes = pass_factory(integer_passes(num_passes * 2))
 
         def reject_even_pass_values(group):