Description of problem:
yum is not updating the system, reporting the next problem:

Traceback (most recent call last):
  File "/usr/share/PackageKit/helpers/yum/yumBackend.py", line 2451, in _runYumTransaction
    rpmDisplay=rpmDisplay)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 5629, in processTransaction
    pkgs = self._downloadPackages(callback)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 5653, in _downloadPackages
    probs = self.downloadPkgs(dlpkgs)
  File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 2147, in downloadPkgs
    self.plugins.run('predownload', pkglist=pkglist)
  File "/usr/lib/python2.7/site-packages/yum/plugins.py", line 187, in run
    func(conduitcls(self, self.base, conf, **kwargs))
  File "/usr/lib/yum-plugins/presto.py", line 633, in predownload_hook
    (problems, more) = downloadPkgs(conduit, pinfo, download_pkgs)
  File "/usr/lib/yum-plugins/presto.py", line 321, in downloadPkgs
    delta = getDelta(po, presto, conduit, conf_mp)
  File "/usr/lib/yum-plugins/presto.py", line 198, in getDelta
    conduit.info(5, -("Could not find delta rpm for package $s.$s.")
TypeError: bad operand type for unary -: 'str'

Version-Release number of selected component (if applicable):
na

How reproducible:
running update manager

Steps to Reproduce:
1. open update manager
2. execute the updating list
3. the update isn't completed due to the error reported
  
Actual results:


Expected results:


Additional info: tryed to update on a terminal and despite doing the workarounds sugested by the system in the terminal, the problem persisted