diff --git a/synapse/api/auth_blocking.py b/synapse/api/auth_blocking.py
index a3f8654128..9c227218e0 100644
--- a/synapse/api/auth_blocking.py
+++ b/synapse/api/auth_blocking.py
@@ -61,8 +61,14 @@ class AuthBlocking:
                 certain blocking reasons like MAU.
 
             requester: If present, and the authenticated entity is a user, checks for
-                presence against existing MAU cohort.
+                presence against existing MAU cohort. Passing in both a `user_id` and
+                `requester` is an error.
         """
+        if requester and user_id:
+            raise Exception(
+                "Passed in both 'user_id' and 'requester' to 'check_auth_blocking'"
+            )
+
         if requester:
             if requester.authenticated_entity.startswith("@"):
                 user_id = requester.authenticated_entity
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 933617cccd..bcb38532e2 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -851,7 +851,7 @@ class UserTokenRestServlet(RestServlet):
         if not self.hs.is_mine_id(user_id):
             raise SynapseError(400, "Only local users can be logged in as")
 
-        body = parse_json_object_from_request(request)
+        body = parse_json_object_from_request(request, allow_empty_body=True)
 
         valid_until_ms = body.get("valid_until_ms")
         if valid_until_ms and not isinstance(valid_until_ms, int):
diff --git a/tests/rest/admin/test_user.py b/tests/rest/admin/test_user.py
index f168367694..c1d7af7c5f 100644
--- a/tests/rest/admin/test_user.py
+++ b/tests/rest/admin/test_user.py
@@ -1759,6 +1759,6 @@ class UserTokenRestTestCase(unittest.HomeserverTestCase):
         )
 
         # Logging in as the other user and joining a room should work, even
-        # though they should be denied.
+        # though the MAU limit would stop the user doing so.
         puppet_token = self._get_token()
         self.helper.join(room_id, user=self.other_user, tok=puppet_token)