--- a/hgext/obsolete.py	Wed Sep 14 19:20:38 2011 +0200
+++ b/hgext/obsolete.py	Wed Sep 14 19:52:57 2011 +0200
@@ -87,7 +87,7 @@
 from mercurial import discovery
 from mercurial import error
 from mercurial import commands
-from mercurial.node import hex, bin, short
+from mercurial.node import hex, bin, short, nullid
 from mercurial.lock import release
 
 ### Patch changectx
@@ -255,6 +255,8 @@
         <subject-full-hex> <object-full-hex>\n"""
     for sub, objs in obssubrels.iteritems():
         for obj in objs:
+            if sub is None:
+                sub = nullid
             flike.write('%s %s\n' % (hex(sub), hex(obj)))
 
 def _obsdeserialise(flike):
@@ -264,7 +266,10 @@
     rels = {}
     for line in flike:
         subhex, objhex = line.split()
-        rels.setdefault(bin(subhex), set()).add(bin(objhex))
+        subnode = bin(subhex)
+        if subnode == nullid:
+            subnode = None
+        rels.setdefault( subnode, set()).add(bin(objhex))
     return rels
 
 def reposetup(ui, repo):
@@ -291,6 +296,8 @@
 
         def addobsolete(self, sub, obj):
             """Add a relation marking that node <sub> is a new version of <obj>"""
+            if sub == nullid:
+                sub = None
             self._obssubrels.setdefault(sub, set()).add(obj)
             self._obsobjrels.setdefault(obj, set()).add(sub)
             try:

--- a/tests/test-obsolete.t	Wed Sep 14 19:20:38 2011 +0200
+++ b/tests/test-obsolete.t	Wed Sep 14 19:52:57 2011 +0200
@@ -265,10 +265,42 @@
 
   $ hg up null
   0 files updated, 0 files merged, 4 files removed, 0 files unresolved
-  $ mkcommit toto # 8
+  $ mkcommit toto # 9
   created new head
-  $ hg debugobsolete 8 0
-  159dfc9fa5d3 try to obsolete immutable changeset 1f0dee641bb7
+  $ hg id -n
+  9
+  $ hg debugobsolete 9 0
+  83b5778897ad try to obsolete immutable changeset 1f0dee641bb7
   $ qlog -r 'obsolete()'
   3
   - 0d3f46688ccc
+allow to just kill changeset
+
+  $ qlog
+  9
+  - 83b5778897ad
+  8
+  - 159dfc9fa5d3
+  4
+  - 725c380fe99b
+  3
+  - 0d3f46688ccc
+  1
+  - 7c3bad9141dc
+  0
+  - 1f0dee641bb7
+
+  $ hg debugobsolete null 9
+  $ hg up null -q # to be not based on 9 anymore
+  $ qlog
+  8
+  - 159dfc9fa5d3
+  4
+  - 725c380fe99b
+  3
+  - 0d3f46688ccc
+  1
+  - 7c3bad9141dc
+  0
+  - 1f0dee641bb7
+