And use buffer(...) there as well

synapse/federation/persistence.py

@@ -99,5 +99,5 @@ class TransactionActions(object):
             transaction.transaction_id,
             transaction.destination,
             response_code,
-            encode_canonical_json(response_dict)
+            response_dict,
         )

synapse/storage/transactions.py

@@ -162,7 +162,8 @@ class TransactionStore(SQLBaseStore):
         return self.runInteraction(
             "delivered_txn",
             self._delivered_txn,
-            transaction_id, destination, code, response_dict
+            transaction_id, destination, code,
+            buffer(encode_canonical_json(response_dict)),
         )
 
     def _delivered_txn(self, txn, transaction_id, destination,