Click on update in point of sales > orders throws exception

In "Point Of Sale"

In Orders

Create a new order.

Click on the "update" link, in order to update the invoice total price

throws

Client Traceback (most recent call last):
  File "/home/odoo/source/web/addons/web/http.py", line 195, in dispatch
    response["result"] = method(self, **self.params)
  File "/home/odoo/source/web/addons/web/controllers/main.py", line 1082, in call_button
    action = self._call_kw(req, model, method, args, {})
  File "/home/odoo/source/web/addons/web/controllers/main.py", line 1070, in _call_kw
    return getattr(req.session.model(model), method)(*args, **kwargs)
  File "/home/odoo/source/web/addons/web/session.py", line 43, in proxy
    result = self.proxy.execute_kw(self.session._db, self.session._uid, self.session._password, self.model, method, args, kw)
  File "/home/odoo/source/web/addons/web/session.py", line 31, in proxy_method
    result = self.session.send(self.service_name, method, *args)
  File "/home/odoo/source/web/addons/web/session.py", line 104, in send
    raise xmlrpclib.Fault(openerp.tools.ustr(e), formatted_info)

Server Traceback (most recent call last):
  File "/home/odoo/source/web/addons/web/session.py", line 90, in send
    return openerp.netsvc.dispatch_rpc(service_name, method, args)
  File "/home/odoo/source/server/openerp/netsvc.py", line 295, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/odoo/source/server/openerp/service/web_services.py", line 614, in dispatch
    res = fn(db, uid, *params)
  File "/home/odoo/source/server/openerp/osv/osv.py", line 169, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  File "/home/odoo/source/server/openerp/osv/osv.py", line 123, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/odoo/source/server/openerp/osv/osv.py", line 179, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/odoo/source/server/openerp/osv/osv.py", line 166, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
AttributeError: 'pos_order' object has no attribute 'button_dummy'