Why does yum update show error as "TypeError: cannot concatenate 'str' and 'NoneType' objects" ?

Issue

• The command yum update shows the following error message.

  # yum update
  Loaded plugins: product-id, rhnplugin, search-disabled-repos, subscription-manager
  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 375, in user_main
      errcode = main(args)
    File "/usr/share/yum-cli/yummain.py", line 170, in main
      base.getOptionsConfig(args)
    File "/usr/share/yum-cli/cli.py", line 263, in getOptionsConfig
      self.conf
    File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 1079, in <lambda>
      conf = property(fget=lambda self: self._getConfig(),
    File "/usr/lib/python2.7/site-packages/yum/__init__.py", line 420, in _getConfig
      self.plugins.run('init')
    File "/usr/lib/python2.7/site-packages/yum/plugins.py", line 188, in run
      func(conduitcls(self, self.base, conf, **kwargs))
    File "/usr/share/yum-plugins/rhnplugin.py", line 81, in init_hook
      proxy_url = get_proxy_url(up2date_cfg)
    File "/usr/share/yum-plugins/rhnplugin.py", line 663, in get_proxy_url
      return proxy_url + netloc
  TypeError: cannot concatenate 'str' and 'NoneType' objects