stack trace if you remove .data after mirror

$ rm -Rf out.d

$ sstream-mirror --max=1 --no-verify exdata/fake/daily/ out.d release=trusty

$ sstream-mirror --max=1 --no-verify exdata/fake/daily/ out.d release=trusty
Traceback (most recent call last):
  File "/usr/bin/sstream-mirror", line 157, in <module>
    main()
  File "/usr/bin/sstream-mirror", line 127, in main
    drmirror.sync(smirror, initial_path)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 89, in sync
    return self.sync_index(reader, path, data, content)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 236, in sync_index
    self.sync(reader, path=epath)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 87, in sync
    return self.sync_products(reader, path, data, content)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 251, in sync_products
    target = self.load_products(path, content_id)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 441, in load_products
    return util.load_content(source.read())
  File "/usr/lib/python3/dist-packages/simplestreams/util.py", line 301, in load_content
    return json.loads(content)
  File "/usr/lib/python3.4/json/__init__.py", line 318, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.4/json/decoder.py", line 343, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.4/json/decoder.py", line 361, in raw_decode
    raise ValueError(errmsg("Expecting value", s, err.value)) from None
ValueError: Expecting value: line 1 column 1 (char 0)

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: simplestreams 0.1.0~bzr401-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Oct 29 08:58:20 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-07-23 (98 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
PackageArchitecture: all
SourcePackage: simplestreams
UpgradeStatus: No upgrade log present (probably fresh install)