diff --git a/addons/account/invoice.py b/addons/account/invoice.py
index e550c33729c..87245488168 100644
--- a/addons/account/invoice.py
+++ b/addons/account/invoice.py
@@ -257,9 +257,15 @@ class account_invoice(osv.osv):
 						res.append(line['id'])
 		return res
 
-	def copy(self, cr, uid, id, default=None, context={}):
-		if not default: default = {}
-		default.update({'state':'draft', 'number':False, 'move_id':False, 'date_invoice': time.strftime('%Y-%m-%d'), 'date_due':False})
+	def copy(self, cr, uid, id, default=None, context=None):
+		if default is None:
+			default = {}
+		default = default.copy()
+		default.update({'state':'draft', 'number':False, 'move_id':False,})
+		if 'date_invoice' not in default:
+			default['date_invoice'] = time.strftime('%Y-%m-%d')
+		if 'date_due' not in default:
+			default['date_due'] = False
 		return super(account_invoice, self).copy(cr, uid, id, default, context)
 
 	def test_paid(self, cr, uid, ids, *args):