diff --git a/base_transaction_id/account_move.py b/base_transaction_id/account_move.py
index dd64e529..2fd2e269 100644
--- a/base_transaction_id/account_move.py
+++ b/base_transaction_id/account_move.py
@@ -33,5 +33,5 @@ class account_move_line(orm.Model):
         if default is None:
             default = {}
         default['transaction_ref'] = False
-        return super(account_move_line, self).\
-            copy_data(cr, uid, id, default=default, context=context)
+        _super = super(account_move_line, self)
+        return _super.copy_data(cr, uid, id, default=default, context=context)
diff --git a/base_transaction_id/invoice.py b/base_transaction_id/invoice.py
index c8a71699..f6fc40ee 100644
--- a/base_transaction_id/invoice.py
+++ b/base_transaction_id/invoice.py
@@ -42,7 +42,7 @@ class AccountInvoice(models.Model):
             if invoice.transaction_id:
                 invoice_account_id = invoice.account_id.id
                 for line in move_lines:
-                    # tuple (0, 0, {values})
+                    # line is a tuple (0, 0, {values})
                     if invoice_account_id == line[2]['account_id']:
                         line[2]['transaction_ref'] = invoice.transaction_id
         return move_lines
diff --git a/base_transaction_id/sale.py b/base_transaction_id/sale.py
index 398eb3c9..159414fb 100644
--- a/base_transaction_id/sale.py
+++ b/base_transaction_id/sale.py
@@ -19,17 +19,15 @@
 #
 ##############################################################################
 
-from openerp.osv.orm import Model
-from openerp.osv import fields
+from openerp.osv import orm, fields
 
 
-class SaleOrder(Model):
+class SaleOrder(orm.Model):
     _inherit = 'sale.order'
 
     _columns = {
         'transaction_id': fields.char(
-            'Transaction id',
-            size=128,
+            'Transaction ID',
             required=False,
             help="Transaction id from the financial institute"),
     }
@@ -42,9 +40,9 @@ class SaleOrder(Model):
         return _super.copy_data(cr, uid, id, default=default, context=context)
 
     def _prepare_invoice(self, cr, uid, order, lines, context=None):
-        # we put the transaction id in the generated invoices
-        invoice_vals = super(SaleOrder, self)._prepare_invoice(
-            cr, uid, order, lines, context=context)
-        invoice_vals.update({
-            'transaction_id': order.transaction_id})
+        """ Propagate the transaction_id from the sale order to the invoice """
+        _super = super(SaleOrder, self)
+        invoice_vals = _super._prepare_invoice(cr, uid, order, lines,
+                                               context=context)
+        invoice_vals['transaction_id'] = order.transaction_id
         return invoice_vals