Try serializing as json rather than msgpack

synapse/handlers/sync.py

@@ -924,9 +924,7 @@ class SyncHandler(object):
                     tags=pagination_config.tags,
                 )
 
-                to_sync_map = {
-                    key: value for key, value in cutoff_list
-                }
+                to_sync_map = {key: value for key, value in cutoff_list}
             else:
                 to_sync_map = {}
 

synapse/types.py

@@ -18,7 +18,7 @@ from synapse.api.errors import SynapseError
 from collections import namedtuple
 
 from unpaddedbase64 import encode_base64, decode_base64
-import msgpack
+import ujson as msgpack
 
 
 Requester = namedtuple("Requester", ["user", "access_token_id", "is_guest"])