resuming is broken in sstream-mirror

$ rm -Rf out.d/ ;
$ sstream-mirror --max=1 http://maas.ubuntu.com/images/ephemeral/releases/streams/v1/index.json out.d/
wait 10 seconds
hit Ctrl-C
...
  File "/usr/lib/python2.7/dist-packages/simplestreams/objectstores/__init__.py", line 113, in insert
    wfp.write(buf)
KeyboardInterrupt

Try it again
$ sstream-mirror --max=1 http://maas.ubuntu.com/images/ephemeral/releases/streams/v1/index.json out.d/
Traceback (most recent call last):
  File "/usr/bin/sstream-mirror", line 93, in <module>
    main()
  File "/usr/bin/sstream-mirror", line 89, in main
    tmirror.sync(smirror, initial_path)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 85, in sync
    return self.sync_index(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 228, in sync_index
    self.sync(reader, path=epath)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 83, in sync
    return self.sync_products(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 302, in sync_products
    self.insert_item(item, src, target, pgree, ipath_cs)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 386, in insert_item
    size=data.get('size'))
  File "/usr/lib/python2.7/dist-packages/simplestreams/objectstores/__init__.py", line 104, in insert
    reader.set_start_pos(os.path.getsize(partfile))
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 149, in set_start_pos
    raise Exception("can't set start pos after open()")
Exception: can't set start pos after open()

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: simplestreams 0.1.0~bzr313-0ubuntu1
ProcVersionSignature: User Name 3.11.0-12.18-generic 3.11.3
Uname: Linux 3.11.0-12-generic x86_64
ApportVersion: 2.12.5-0ubuntu1
Architecture: amd64
Date: Wed Oct 9 20:37:38 2013
Ec2AMI: ami-000005a3
Ec2AMIManifest: FIXME
Ec2AvailabilityZone: nova
Ec2InstanceType: m1.large
Ec2Kernel: aki-0000022a
Ec2Ramdisk: ari-0000022a
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: simplestreams
UpgradeStatus: No upgrade log present (probably fresh install)