diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c762b58902c8..55787563c001 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -243,7 +243,14 @@ def __init__(self, hs):
 
         self.spam_checker = hs.get_spam_checker()
 
-        if self.config.block_events_without_consent_error is not None:
+        self._block_events_without_consent_error = (
+            self.config.block_events_without_consent_error
+        )
+
+        # we need to construct a ConsentURIBuilder here, as it checks that the necessary
+        # config options, but *only* if we have a configuration for which we are
+        # going to need it.
+        if self._block_events_without_consent_error:
             self._consent_uri_builder = ConsentURIBuilder(self.config)
 
     @defer.inlineCallbacks
@@ -378,7 +385,7 @@ def assert_accepted_privacy_policy(self, requester):
         Raises:
             ConsentNotGivenError: if the user has not given consent yet
         """
-        if self.config.block_events_without_consent_error is None:
+        if self._block_events_without_consent_error is None:
             return
 
         # exempt AS users from needing consent
@@ -405,7 +412,7 @@ def assert_accepted_privacy_policy(self, requester):
         consent_uri = self._consent_uri_builder.build_user_consent_uri(
             requester.user.localpart,
         )
-        msg = self.config.block_events_without_consent_error % {
+        msg = self._block_events_without_consent_error % {
             'consent_uri': consent_uri,
         }
         raise ConsentNotGivenError(
diff --git a/tests/handlers/test_register.py b/tests/handlers/test_register.py
index c9c150627305..ef6ad127fcd8 100644
--- a/tests/handlers/test_register.py
+++ b/tests/handlers/test_register.py
@@ -187,6 +187,12 @@ def test_auto_create_auto_join_rooms_when_support_user_exists(self):
 
     @defer.inlineCallbacks
     def test_auto_create_auto_join_where_no_consent(self):
+        """XXX what is this trying to test? I *think* it is trying to test
+        that we are not auto-joined to the server notices room if
+        block_events_without_consent_error is set, but (a) that doesn't seem to be
+        a sensible thing to test for, and (b) it doesn't work anyway because you can't
+        change the config after the EventCreationHandler has been instantiated.
+        """
         self.hs.config.user_consent_at_registration = True
         self.hs.config.block_events_without_consent_error = "Error"
         room_alias_str = "#room:test"