V11 CE Ubuntu 16.04

When Validate closing posting entries in POS, I get the following error:

Error:
Odoo Server Error

Traceback (most recent call last):
  File "/odoo/odoo-server/odoo/http.py", line 650, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/odoo/odoo-server/odoo/http.py", line 310, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/odoo/odoo-server/odoo/tools/pycompat.py", line 87, in reraise
    raise value
  File "/odoo/odoo-server/odoo/http.py", line 692, in dispatch
    result = self._call_function(**self.params)
  File "/odoo/odoo-server/odoo/http.py", line 342, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/odoo/odoo-server/odoo/service/model.py", line 97, in wrapper
    return f(dbname, *args, **kwargs)
  File "/odoo/odoo-server/odoo/http.py", line 335, in checked_call
    result = self.endpoint(*a, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 936, in __call__
    return self.method(*args, **kw)
  File "/odoo/odoo-server/odoo/http.py", line 515, in response_wrap
    response = f(*args, **kw)
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 938, in call_button
    action = self._call_kw(model, method, args, {})
  File "/odoo/odoo-server/addons/web/controllers/main.py", line 926, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 689, in call_kw
    return call_kw_multi(method, model, args, kwargs)
  File "/odoo/odoo-server/odoo/api.py", line 680, in call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/odoo/odoo-server/addons/point_of_sale/models/pos_session.py", line 257, in action_pos_session_closing_control
    session.action_pos_session_close()
  File "/odoo/odoo-server/addons/point_of_sale/models/pos_session.py", line 285, in action_pos_session_close
    self.with_context(ctx)._confirm_orders()
  File "/odoo/odoo-server/addons/point_of_sale/models/pos_session.py", line 29, in _confirm_orders
    orders.with_context(force_company=company_id)._create_account_move_line(session, move)
  File "/odoo/odoo-server/addons/point_of_sale/models/pos_order.py", line 421, in _create_account_move_line
    add_anglosaxon_lines(grouped_data)
  File "/odoo/odoo-server/addons/point_of_sale/models/pos_order.py", line 277, in add_anglosaxon_lines
    line1 = Product._convert_prepared_anglosaxon_line(line1, order.partner_id)
AttributeError: 'product.product' object has no attribute '_convert_prepared_anglosaxon_line'

Any help would be greatly appreciated.

thanks,

dave