RHUA server not able to sync specific repository from CDN, sync fails with error "TypeError: 'NoneType' object is unsubscriptable"

Issue

• In rhui-manager status command output, there is issue while syncing specific repository:

Red Hat Enterprise Linux 6 Server from RHUI (RPMs) (6Server-x86_64)
02-10-2016 13:51             12-17-2015 20:40             Error

Error message in pulp.log:

2016-02-10 08:39:31,298 12566:139645125904128: pulp.server.api.repo_sync:INFO: repo_sync:275 Sync returned 16605 packages, 3320 errata in 2889.87199497 seconds
2016-02-10 08:39:34,769 12566:139645125904128: pulp.server.tasking.task:ERROR: task:478 Task failed: Task ff68f4a1-ccda-11e5-b523-005056ae4ef8: _sync(rhel-6-server-rhui-rpms-6Server-x86_64, synchronizer=<pulp.server.api.synchronizers.YumSynchronizer object at 0x7f0206246190>, skip=SON([(u'distribution', 1)]), progress_callback=<bound method RepoSyncTask.progress_callback of <pulp.server.api.repo_sync_task.RepoSyncTask object at 0x7f0206246050>>)
Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/pulp/server/tasking/task.py", line 424, in run
    result = self.callable(*self.args, **self.kwargs)
  File "/usr/lib/python2.6/site-packages/pulp/server/api/repo_sync.py", line 282, in _sync
    old_pkgs = filter(lambda pkg: pkg["repo_defined"], old_pkgs)
  File "/usr/lib/python2.6/site-packages/pulp/server/api/repo_sync.py", line 282, in <lambda>
    old_pkgs = filter(lambda pkg: pkg["repo_defined"], old_pkgs)
TypeError: 'NoneType' object is unsubscriptable