Description of problem:
dnf crashed when executing 'dnf -v repolist all'.

Version-Release number of selected component (if applicable):
dnf-0.5.2-1.fc20.noarch

Additional info:
Config time: 0.007
cachedir: /var/cache/dnf/x86_64/20
dnf version: 0.5.2
repo: using cache for: fedora
not found deltainfo for: Fedora 20 - x86_64
repo: using cache for: rpmfusion-free-updates
not found deltainfo for: RPM Fusion for Fedora 20 - Free - Updates
repo: using cache for: adobe-linux-x86_64
not found deltainfo for: Adobe Systems Incorporated
repo: using cache for: rpmfusion-nonfree-updates
not found deltainfo for: RPM Fusion for Fedora 20 - Nonfree - Updates
repo: using cache for: rpmfusion-free
not found deltainfo for: RPM Fusion for Fedora 20 - Free
repo: using cache for: updates
repo: using cache for: rpmfusion-nonfree
not found deltainfo for: RPM Fusion for Fedora 20 - Nonfree
hawkey sack setup time: 0.453
Repo-id      : adobe-linux-x86_64
Repo-name    : Adobe Systems Incorporated
Repo-status  : enabled
Repo-updated : Wed Jun  4 04:01:09 2014
Repo-pkgs    : 2
Repo-size    : 6.9 M
Repo-baseurl : http://linuxdownload.adobe.com/linux/x86_64/
Repo-expire  : 172800 second(s) (last: Sun Jul  6 17:21:07 2014)
Repo-filename: /etc/yum.repos.d/adobe-linux-x86_64.repo

Repo-id      : fedora
Repo-name    : Fedora 20 - x86_64
Repo-status  : enabled
Repo-revision: 1386924430
Repo-tags    : binary-x86_64
Repo-distro-tags: [cpe:/o:fedoraproject:fedora:20]: N, l, l, u
Repo-updated : Fri Dec 13 17:55:41 2013
Repo-pkgs    : 38597
Repo-size    : 38 G
Repo-metalink: https://mirrors.fedoraproject.org/metalink?repo=fedora-20&arch=x86_64
  Updated    : Sun Jul  6 17:20:16 2014
Repo-baseurl : rsync://ftp.jaist.ac.jp/pub/Linux/Fedora/releases/20/Everything/x86_64/os/ (49 more)
Repo-expire  : 604800 second(s) (last: Sun Jul  6 17:20:16 2014)
Repo-filename: /etc/yum.repos.d/fedora.repo

Cleaning up.
Traceback (most recent call last):
  File "/usr/bin/dnf", line 36, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 206, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 82, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 132, in _main
    cli.run()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1163, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf/cli/commands/__init__.py", line 638, in run
    last = time.ctime(md.timestamp)
AttributeError: 'NoneType' object has no attribute 'timestamp'