Hmm, there's no such change?

I think I figured out a repro case anyway, this one on a mac, latest version 
osx and hg. The key is to set a bookmark to a change and then rollback that 
change. The bookmark now points to a non-existing change.

Dumbledore:htest axl$ hg init a
Dumbledore:htest axl$ cd a
Dumbledore:a axl$ echo foo>foo
Dumbledore:a axl$ hg add
adding foo
Dumbledore:a axl$ hg ci -m "foo"
Dumbledore:a axl$ echo bar>>foo
Dumbledore:a axl$ hg ci -m "foo2"
Dumbledore:a axl$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
Dumbledore:a axl$ hg tag 1.0
Dumbledore:a axl$ hg bookmark 1.0
Dumbledore:a axl$ hg rollback
rolling back last transaction
Dumbledore:a axl$ hg forget .hgtags 
Dumbledore:a axl$ rm .hgtags 
Dumbledore:a axl$ hg bookmarks
no bookmarks set
Dumbledore:a axl$ hg bookmark master
** unknown exception encountered, details follow
** report bug details to http://mercurial.selenic.com/bts/
** or mercurial@selenic.com
** Mercurial Distributed SCM (version 1.4.2+20100104)
** Extensions loaded: bookmarks, graphlog
Traceback (most recent call last):
  File "/usr/local/bin/hg", line 27, in <module>
    mercurial.dispatch.run()
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 16, 
in run
    sys.exit(dispatch(sys.argv[1:]))
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 30, 
in dispatch
    return _runcatch(u, args)
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 46, 
in _runcatch
    return _dispatch(ui, args)
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 454, 
in _dispatch
    return runcommand(lui, repo, cmd, fullargs, ui, options, d)
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 324, 
in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 505, 
in _runcommand
    return checkargs()
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 459, 
in checkargs
    return cmdfunc()
  File "/Library/Python/2.6/site-packages/mercurial/dispatch.py", line 453, 
in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
  File "/Library/Python/2.6/site-packages/mercurial/util.py", line 386, in 
check
    return func(*args, **kwargs)
  File "/Library/Python/2.6/site-packages/hgext/bookmarks.py", line 172, in 
bookmark
    if ((mark in repo.branchtags() or mark == repo.dirstate.branch())
  File "/Library/Python/2.6/site-packages/mercurial/localrepo.py", line 344, 
in branchtags
    for bn, heads in self.branchmap().iteritems():
AttributeError: 'NoneType' object has no attribute 'iteritems'
Dumbledore:a axl$