Purchase Order: TypeError: exec: arg 1 must be a string, file, or code object

add a line to a Purchase Order.
All data displayed OK
Changed Qty from 6 to 12
Selected Tax code zero value (python setting)
Saved

Then this request came up

Maybe something to do with Python Tax setting

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Traceback (most recent call last):
  File "/home/odoo/openerp/server/bin/netsvc.py", line 244, in dispatch
    result = LocalService(service_name)(method, *params)
  File "/home/odoo/openerp/server/bin/netsvc.py", line 73, in __call__
    return getattr(self, method)(*params)
  File "/home/odoo/openerp/server/bin/addons/use_control/services.py", line 56, in execute
    return super(recording_objects_proxy, self).execute(db, uid, passwd, object, method, *args)
  File "/home/odoo/openerp/server/bin/service/web_services.py", line 576, in execute
    res = service.execute(db, uid, object, method, *args)
  File "/home/odoo/openerp/server/bin/osv/osv.py", line 58, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/odoo/openerp/server/bin/osv/osv.py", line 119, in execute
    res = pool.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/odoo/openerp/server/bin/osv/osv.py", line 111, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/odoo/openerp/server/bin/osv/orm.py", line 2710, in write
    self.pool.get(object)._store_set_values(cr, user, ids, fields, context)
  File "/home/odoo/openerp/server/bin/osv/orm.py", line 2917, in _store_set_values
    result = self._columns[val[0]].get(cr, self, ids, val, uid, context=context)
  File "/home/odoo/openerp/server/bin/osv/fields.py", line 650, in get
    res = self._fnct(obj, cr, user, ids, name, self._arg, context)
  File "/home/odoo/openerp/server/bin/addons/purchase/purchase.py", line 58, in _amount_all
    for c in self.pool.get('account.tax').compute(cr, uid, line.taxes_id, line.price_unit, line.product_qty, order.partner_address_id.id, line.product_id, order.partner_id):
  File "/home/odoo/openerp/server/bin/addons/account/account.py", line 1439, in compute
    res = self._unit_compute(cr, uid, taxes, price_unit, address_id, product, partner, quantity)
  File "/home/odoo/openerp/server/bin/addons/account/account.py", line 1363, in _unit_compute
    taxes = self._applicable(cr, uid, taxes, price_unit, address_id, product, partner)
  File "/home/odoo/openerp/server/bin/addons/account/account.py", line 1355, in _applicable
    exec tax.python_applicable in localdict
TypeError: exec: arg 1 must be a string, file, or code object

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This is the first time I have raised a P.O I have tried several times ensuring that the price lists are OK, Warehouse is OK Locations are OK. Each time the save fails