--- a/hgext/obsolete.py	Wed Sep 21 03:49:59 2011 +0200
+++ b/hgext/obsolete.py	Wed Sep 21 03:52:13 2011 +0200
@@ -339,9 +339,14 @@
             self._obsobjrels.setdefault(obj, set()).add(sub)
             try:
                 if not self.nodestate(obj).mutable:
-                    self.ui.warn(
-                        _("%(sub)s try to obsolete immutable changeset %(obj)s\n")
-                        % {'sub': short(sub), 'obj': short(obj)})
+                    if sub is None:
+                        self.ui.warn(
+                            _("trying to kill immutable changeset %(obj)s\n")
+                            % {'obj': short(obj)})
+                    if sub is not None:
+                        self.ui.warn(
+                            _("%(sub)s try to obsolete immutable changeset %(obj)s\n")
+                            % {'sub': short(sub), 'obj': short(obj)})
                 self.changelog.hiddenrevs.add(repo[obj].rev())
             except (error.RepoLookupError, error.LookupError):
                 pass #unknow revision (but keep propagating the data