obnam forget raises a KeyError

cmdline:
obnam forget --keep=24h,7d,4w,12m

output:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cliapp/app.py", line 169, in _run
    self.process_args(args)
  File "/usr/lib/python2.7/dist-packages/obnamlib/app.py", line 170, in process_args
    cliapp.Application.process_args(self, args)
  File "/usr/lib/python2.7/dist-packages/cliapp/app.py", line 416, in process_args
    method(args[1:])
  File "/usr/lib/python2.7/dist-packages/obnamlib/plugins/forget_plugin.py", line 76, in forget
    self.repo.commit_client()
  File "/usr/lib/python2.7/dist-packages/obnamlib/repo.py", line 473, in commit_client
    self._really_remove_generations(self.removed_generations)
  File "/usr/lib/python2.7/dist-packages/obnamlib/repo.py", line 573, in _really_remove_generations
    remove_gens(remove_genids)
  File "/usr/lib/python2.7/dist-packages/obnamlib/repo.py", line 560, in remove_gens
    self.client.remove_generation(genid)
  File "/usr/lib/python2.7/dist-packages/obnamlib/clientmetadatatree.py", line 292, in remove_generation
    self.forest.remove_tree(tree)
  File "/usr/lib/python2.7/dist-packages/larch/forest.py", line 125, in remove_tree
    tree._decrement(tree.root.id)
  File "/usr/lib/python2.7/dist-packages/larch/tree.py", line 601, in _decrement
    self._decrement(child_id)
  File "/usr/lib/python2.7/dist-packages/larch/tree.py", line 589, in _decrement
    refcount = self.node_store.get_refcount(node_id)
  File "/usr/lib/python2.7/dist-packages/larch/nodestore_disk.py", line 271, in get_refcount
    return self.rs.get_refcount(node_id)
  File "/usr/lib/python2.7/dist-packages/larch/refcountstore.py", line 76, in get_refcount
    return self.refcounts[node_id]
KeyError: 126859