#7 — Linguaplone references lost

The error occured during Update Schema (archetypes_tool) on a custom AT content type.
After updateing the schema, all Linguaplone references are lost.

The same behaviour seems to apply to the build in ATDocument .

Environment:
Archetypes 1.3.7-final
Zope 2.7.8-final
Plone 2.1.2
Python 2.3.5
Windows XP

My content type:
 from AccessControl import ClassSecurityInfo
 from Products.Archetypes.atapi import *
 try:
     from Products.LinguaPlone.public import *
 except ImportError:
     HAS_LINGUAPLONE = False
 else:
     HAS_LINGUAPLONE = True

 from Products.Employee.config import *

 schema=Schema((
     StringField('id',
         widget=StringWidget
         (
             label="id",
             description="lastname_firstname (lower case letters only!)",
             label_msgid='Employee_label_id',
             description_msgid='Employee_help_id',
             i18n_domain='Employee',
         ),
         required=1,
         searchable=1
     ),

     StringField('title',
         widget=StringWidget
         (
             label="Title",
             description="Lastname, Firstname",
             label_msgid='Employee_label_title',
             description_msgid='Employee_help_title',
             i18n_domain='Employee',
         ),
         languageindependent="1",
         searchable=1,
         required=1
     ),

     StringField('Position',
         index="FieldIndex",
         widget=StringWidget
         (
             label="Position",
             description="For example Division Head, Secretary, etc",
             label_msgid='Employee_label_Position',
             description_msgid='Employee_help_Position',
             i18n_domain='Employee',
         ),
         required=0,
         searchable=1
     ),

     StringField('MobileNumber',
         widget=StringWidget
         (
             label="Mobile number",
             label_msgid='Employee_label_MobileNumber',
             description_msgid='Employee_help_MobileNumber',
             i18n_domain='Employee',
         ),
         languageindependent="1",
         searchable=1,
         required=0
     ),



 ),
 )

 Employee_schema = BaseSchema + \
     schema

 class Employee(BaseContent):
     security = ClassSecurityInfo()
     __implements__ = (getattr(BaseContent,'__implements__',()),)


     # This name appears in the 'add' box
     archetype_name = 'Employee'

     meta_type = 'Employee'
     portal_type = 'Employee'
     allowed_content_types = []
     filter_content_types = 0
     global_allow = 1
     allow_discussion = 0
     #content_icon = 'Employee.gif'
     immediate_view = 'base_view'
     default_view = 'base_view'
     suppl_views = ()
     typeDescription = "Employee"
     typeDescMsgId = 'description_edit_employee'

     schema = Employee_schema

     security.declarePublic('Title')
     def Title(self):
         """
        """
        return self.getTitle()


 registerType(Employee,PROJECTNAME)