diff --git a/acme/acme/messages.py b/acme/acme/messages.py
index 4b4fa5003..477e15de2 100644
--- a/acme/acme/messages.py
+++ b/acme/acme/messages.py
@@ -250,6 +250,7 @@ class Registration(ResourceBody):
     contact = jose.Field('contact', omitempty=True, default=())
     agreement = jose.Field('agreement', omitempty=True)
     status = jose.Field('status', omitempty=True)
+    terms_of_service_agreed = jose.Field('terms-of-service-agreed', omitempty=True)
 
     phone_prefix = 'tel:'
     email_prefix = 'mailto:'