diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -277,7 +277,7 @@ def _notransaction():
     to be created"""
     raise TransactionUnavailable()
 
-def processbundle(repo, unbundler, transactiongetter=_notransaction):
+def processbundle(repo, unbundler, transactiongetter=None):
     """This function process a bundle, apply effect to/from a repo
 
     It iterates over each part then searches for and uses the proper handling
@@ -288,6 +288,8 @@ def processbundle(repo, unbundler, trans
 
     Unknown Mandatory part will abort the process.
     """
+    if transactiongetter is None:
+        transactiongetter = _notransaction
     op = bundleoperation(repo, transactiongetter)
     # todo:
     # - replace this is a init function soon.