cubicweb-file #1207305 migration crash [open]

while upgrading from cw 3.8 to 3.9:

Execute command: rename_entity_type('Image', 'File') ? [Y/n/a(bort)/s(hell)]:
Traceback (most recent call last):
 File "/home/aurelien/src/fcw/cubicweb/", line 147, in run_arg
   status = cmdmeth(appid)
 File "/home/aurelien/src/fcw/cubicweb/", line 743, in upgrade_instance
   mih.migrate(vcconf, reversed(toupgrade), self.config)
 File "/home/aurelien/src/fcw/cubicweb/server/", line 117, in migrate
   super(ServerMigrationHelper, self).migrate(vcconf, toupgrade, options)
 File "/home/aurelien/src/fcw/cubicweb/", line 178, in migrate
 File "/home/aurelien/src/fcw/cubicweb/server/", line 122, in cmd_process_script
   migrscript, funcname, *args, **kwargs)
 File "/home/aurelien/src/fcw/cubicweb/", line 330, in cmd_process_script
   execfile(migrscript, scriptlocals)
 File "/home/aurelien/src/fcw/cubes/file/migration/", line 1, in <module>
   rename_entity_type('Image', 'File')
 File "/home/aurelien/src/fcw/cubicweb/", line 134, in <lambda>
 File "/home/aurelien/src/fcw/cubicweb/", line 200, in interact
   return meth(*args, **kwargs)
 File "/home/aurelien/src/fcw/cubicweb/server/", line 891, in cmd_rename_entity_type
 File "/home/aurelien/src/fcw/cubicweb/server/", line 265, in commit
 File "/home/aurelien/src/fcw/cubicweb/", line 669, in commit
   return self._repo.commit(self.sessionid, **self._txid())
 File "/home/aurelien/src/fcw/cubicweb/server/", line 650, in commit
   return session.commit()
 File "/home/aurelien/src/fcw/cubicweb/server/", line 746, in commit
   operation.handle_event('%s_event' % trstate)
 File "/home/aurelien/src/fcw/cubicweb/server/", line 431, in handle_event
   getattr(self, event)()
 File "/home/aurelien/src/fcw/cubicweb/hooks/", line 99, in precommit_event
   raise ValidationError(eid, {role_name(rtype, self.role): msg})
ValidationError: 601 (to_entity-subject): at least one relation to entity is required on Relation (601)
instance forge not upgraded: 601 (to_entity-subject): at least one relation to entity is required on Relation (601)
done in<not specified>
closed by<not specified>