Invalid literal for int() in sequence with prefix

V6RC1 with client GTK under windows seven.
Openerp-server installeed on a debian 64 machine

Prb : I've modified the sequence for the Packs objects specifying a prefix with string : "UM"
Know when I create a new pack, i have the following error :

Environment Information :
System : Windows-32bit
OS Name : nt
Operating System Release :
Operating System Version : 32bit
Operating System Architecture : 32bit
Operating System Locale : fr_FR.cp1252
Python Version : 2.5.2
OpenERP-Client Version : 6.0.0-rc2-test
Last revision No. & ID :Bazaar Package not Found !Traceback (most recent call last):
  File "/openerp/openerp-server-6.0.0-rc1/bin/netsvc.py", line 489, in dispatch
    result = ExportService.getService(service_name).dispatch(method, auth, params)
  File "/openerp/openerp-server-6.0.0-rc1/bin/service/web_services.py", line 586, in dispatch
    res = fn(db, uid, *params)
  File "/openerp/openerp-server-6.0.0-rc1/bin/osv/osv.py", line 57, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/openerp/openerp-server-6.0.0-rc1/bin/osv/osv.py", line 140, in execute
    res = pool.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/openerp/openerp-server-6.0.0-rc1/bin/osv/osv.py", line 130, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/openerp/openerp-server-6.0.0-rc1/bin/osv/orm.py", line 1070, in default_get
    defaults[f] = self._defaults[f](self, cr, uid, context)
  File "/openerp/openerp-server-6.0.0-rc1/bin/addons/stock/stock.py", line 472, in make_sscc
    return sequence + str(self.checksum(sequence))
  File "/openerp/openerp-server-6.0.0-rc1/bin/addons/stock/stock.py", line 466, in checksum
    sum += int(sscc_part) * int(salt_part)
ValueError: invalid literal for int() with base 10: 'U'