diff --git a/acme/acme/challenges.py b/acme/acme/challenges.py
index 0f25f771b..1b4f6c2ed 100644
--- a/acme/acme/challenges.py
+++ b/acme/acme/challenges.py
@@ -345,7 +345,7 @@ class TLSSNI01Response(KeyAuthorizationChallengeResponse):
     def z(self):
         """``z`` value used for verification."""
         return hashlib.sha256(
-            self.key_authorization.encode("utf-8")).hexdigest().encode()
+            self.key_authorization.encode("utf-8")).hexdigest().lower().encode()
 
     @property
     def z_domain(self):