Fedora 20 - AssertionError on yum update / yum install

When I run yum update or yum install, I receive the following error:

Traceback (most recent call last):
  File "/bin/yum", line 29, in <module>
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 355, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 261, in main
    return_code = base.doTransaction()
  File "/usr/share/yum-cli/cli.py", line 670, in doTransaction
    problems = self.downloadPkgs(downloadpkgs, callback_total=self.download_callback_total_cb)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 2434, in downloadPkgs
    presto.rebuild(po)
  File "/usr/lib/python2.7/site-packages/yum/drpm.py", line 289, in rebuild
    self.dequeue_max()
  File "/usr/lib/python2.7/site-packages/yum/drpm.py", line 325, in dequeue_max
    if not self.dequeue(block=False):
  File "/usr/lib/python2.7/site-packages/yum/drpm.py", line 331, in dequeue
    self._wait()
  File "/usr/lib/python2.7/site-packages/yum/drpm.py", line 259, in _wait
    assert pid in self.jobs
AssertionError

For more information, see: http://ift.tt/1CRZFFl

How can I fix this?