Buildbot nightly build is failing to checkout the branch - gerrit

Question

I am using buildbot for building and testing chromium source. We have a local repo maintained for chromium source. We are using Gerrit for maintaining the repository. The buildbot gets successfully triggered when some change is checked in.
I am facing problem with Nightly builds. I want the buildbot to get the source and build, even when there is no change. But, it fails with the following error:
===Running git checkout --force origin/ ===
error: pathspec 'origin/' did not match any file(s) know to git.
===Failed in 0.0. mins===
Can somebody tell me how to get this working?
My master.cfg is as below:
from master import master_utils
from master import slaves_list
from buildbot.schedulers import timed
from buildbot.changes import filter
import config
import master_site_config
ActiveMaster = master_site_config.Chromium
c = BuildmasterConfig = {}
c['change_source'] = []
c['schedulers'] = []
c['status'] = [] pendingRequests = {}
c['builders'] = []
import master_source_cfg
import master_full_cfg
master_source_cfg.Update(config, ActiveMaster, c)
master_full_cfg.Update(config, ActiveMaster, c)
c['logCompressionLimit'] = False
c['projectName'] = ActiveMaster.project_name
c['projectURL'] = config.Master.project_url
c['buildbotURL'] = 'build.chromium.org/p/chromium/';
slaves = slaves_list.SlavesList('slaves.cfg', 'Chromium')
# Trying to find the location of adding factories to the builder
from twisted.python import log
for builder in c['builders']:
log.msg('My BUILDER',builder)
log.msg(dir(builder['factory']))
builder['slavenames'] = slaves.GetSlavesName(builder=builder['name'])
c['slaves'] = master_utils.AutoSetupSlaves(
c['builders'],
config.Master.GetBotPassword(),
missing_recipients=['buildbot#chromium-build-health.appspotmail.com'])
master_utils.VerifySetup(c, slaves)
master_utils.AutoSetupMaster(c, ActiveMaster, enable_http_status_push=ActiveMaster.is_production_host)
c['buildHorizon'] = 3000
c['logHorizon'] = 3000
c['eventHorizon'] = 200
I had missed to add the last part of the master.cfg which contains the configuration for nightly:
c['schedulers'].append(
timed.Nightly(name='nightly',
branch='src',
change_filter=filter.ChangeFilter(project_re='pj/Sample/chromium.*', branch=['buildbot-testing1']),
builderNames=['Linux x64'],
hour=8,
minute=15))
With the above configuration, the nightly gets triggered at the correct time.
But, it is not checking out the 'buildbot-testing1' branch as needed.
Instead, it exits with the following error:
===Running git checkout --force origin/ ===
error: pathspec 'origin/' did not match any file(s) know to git.
===Failed in 0.0. mins===