diff --git a/synapse/federation/persistence.py b/synapse/federation/persistence.py
index 76a9dcd77..865766eb2 100644
--- a/synapse/federation/persistence.py
+++ b/synapse/federation/persistence.py
@@ -23,8 +23,6 @@ from twisted.internet import defer
 
 from synapse.util.logutils import log_function
 
-from syutil.jsonutil import encode_canonical_json
-
 import logging
 
 
@@ -71,7 +69,7 @@ class TransactionActions(object):
             transaction.transaction_id,
             transaction.origin,
             code,
-            encode_canonical_json(response)
+            response,
         )
 
     @defer.inlineCallbacks
diff --git a/synapse/storage/transactions.py b/synapse/storage/transactions.py
index 89dd7d894..b5b21a9b1 100644
--- a/synapse/storage/transactions.py
+++ b/synapse/storage/transactions.py
@@ -17,6 +17,7 @@ from ._base import SQLBaseStore, cached
 
 from collections import namedtuple
 
+from syutil.jsonutil import encode_canonical_json
 import logging
 
 logger = logging.getLogger(__name__)
@@ -82,7 +83,7 @@ class TransactionStore(SQLBaseStore):
                 "transaction_id": transaction_id,
                 "origin": origin,
                 "response_code": code,
-                "response_json": response_dict,
+                "response_json": buffer(encode_canonical_json(response_dict)),
             },
             or_ignore=True,
             desc="set_received_txn_response",