1 files changed, 3 insertions, 2 deletions
diff --git a/synapse/federation/units.py b/synapse/federation/units.py
index b662c42621..0f8bf000ac 100644
--- a/synapse/federation/units.py
+++ b/synapse/federation/units.py
@@ -18,6 +18,7 @@ server protocol.
 """
 
 import logging
+from typing import Optional
 
 import attr
 
@@ -98,7 +99,7 @@ class Transaction(JsonEncodedObject):
         "pdus",
     ]
 
-    def __init__(self, transaction_id=None, pdus=[], **kwargs):
+    def __init__(self, transaction_id=None, pdus: Optional[list] = None, **kwargs):
         """If we include a list of pdus then we decode then as PDU's
         automatically.
         """
@@ -107,7 +108,7 @@ class Transaction(JsonEncodedObject):
         if "edus" in kwargs and not kwargs["edus"]:
             del kwargs["edus"]
 
-        super().__init__(transaction_id=transaction_id, pdus=pdus, **kwargs)
+        super().__init__(transaction_id=transaction_id, pdus=pdus or [], **kwargs)
 
     @staticmethod
     def create_new(pdus, **kwargs):