diff --git a/tests/crypto/test_event_signing.py b/tests/crypto/test_event_signing.py
index e1800da65a95..a72a0103d3b8 100644
--- a/tests/crypto/test_event_signing.py
+++ b/tests/crypto/test_event_signing.py
@@ -36,6 +36,9 @@
 
 class EventSigningTestCase(unittest.TestCase):
     def setUp(self):
+        # NB: `signedjson` expects `nacl.signing.SigningKey` instances which have been
+        # monkeypatched to include new `alg` and `version` attributes. This is captured
+        # by the `signedjson.types.SigningKey` protocol.
         self.signing_key: signedjson.types.SigningKey = nacl.signing.SigningKey(
             SIGNING_KEY_SEED
         )