diff --git a/hgext/convert/common.py b/hgext/convert/common.py
index c49be208ae34e4437e3e808f9e4b2d68916f4834_aGdleHQvY29udmVydC9jb21tb24ucHk=..1404146157d995dbb154473647c82762ba1c7d74_aGdleHQvY29udmVydC9jb21tb24ucHk= 100644
--- a/hgext/convert/common.py
+++ b/hgext/convert/common.py
@@ -55,7 +55,7 @@
 
 class commit(object):
     def __init__(self, author, date, desc, parents, branch=None, rev=None,
-                 extra={}, sortkey=None, saverev=True, phase=phases.draft,
+                 extra=None, sortkey=None, saverev=True, phase=phases.draft,
                  optparents=None):
         self.author = author or 'unknown'
         self.date = date or '0 0'
@@ -64,7 +64,7 @@
         self.optparents = optparents or [] # will be used if already converted
         self.branch = branch
         self.rev = rev
-        self.extra = extra
+        self.extra = extra or {}
         self.sortkey = sortkey
         self.saverev = saverev
         self.phase = phase