This happens somewhat frequently recently. Here's an example:

 04-26-2017 [00:53:31] Output below this line is for buildbot consumption:
Will return from run_suite with status: OK
00:53:46: INFO: RunCommand: /b/cbuild/repository/chromite/third_party/swarming.client/swarming.py run --swarming chromeos-proxy.appspot.com --task-summary-json /tmp/cbuildbot-tmp3nqkpT/tmpVkDqNP/temp_summary.json --raw-cmd --task-name veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity --dimension os Ubuntu-14.04 --dimension pool default --print-status-updates --timeout 14400 --io-timeout 14400 --hard-timeout 14400 --expiration 1200 '--tags=priority:PFQ' '--tags=suite:sanity' '--tags=build:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1' '--tags=task_name:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity' '--tags=board:veyron_minnie' -- /usr/local/autotest/site_utils/run_suite.py --build veyron_minnie-chrome-pfq/R60-9496.0.0-rc1 --board veyron_minnie --suite_name sanity --pool bvt --num 1 --file_bugs True --priority PFQ --timeout_mins 180 --retry False --minimum_duts 1 --suite_min_duts 1 --offload_failures_only False --job_keyvals "{'datastore_parent_key': ('Build', 1475832, 'BuildStage', 43355589L)}" --json_dump -m 114037141
01:14:06: WARNING: Exception is not retriable return code: 1; command: /b/cbuild/repository/chromite/third_party/swarming.client/swarming.py run --swarming chromeos-proxy.appspot.com --task-summary-json /tmp/cbuildbot-tmp3nqkpT/tmpVkDqNP/temp_summary.json --raw-cmd --task-name veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity --dimension os Ubuntu-14.04 --dimension pool default --print-status-updates --timeout 14400 --io-timeout 14400 --hard-timeout 14400 --expiration 1200 '--tags=priority:PFQ' '--tags=suite:sanity' '--tags=build:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1' '--tags=task_name:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity' '--tags=board:veyron_minnie' -- /usr/local/autotest/site_utils/run_suite.py --build veyron_minnie-chrome-pfq/R60-9496.0.0-rc1 --board veyron_minnie --suite_name sanity --pool bvt --num 1 --file_bugs True --priority PFQ --timeout_mins 180 --retry False --minimum_duts 1 --suite_min_duts 1 --offload_failures_only False --job_keyvals "{'datastore_parent_key': ('Build', 1475832, 'BuildStage', 43355589L)}" --json_dump -m 114037141
Priority was reset to 100
Triggered task: veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity
Waiting for results from the following shards: 0
N/A: 35c21201ea00fd10 None

cmd=['/b/cbuild/repository/chromite/third_party/swarming.client/swarming.py', 'run', '--swarming', 'chromeos-proxy.appspot.com', '--task-summary-json', '/tmp/cbuildbot-tmp3nqkpT/tmpVkDqNP/temp_summary.json', '--raw-cmd', '--task-name', u'veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity', '--dimension', 'os', 'Ubuntu-14.04', '--dimension', 'pool', 'default', '--print-status-updates', '--timeout', '14400', '--io-timeout', '14400', '--hard-timeout', '14400', '--expiration', '1200', u'--tags=priority:PFQ', u'--tags=suite:sanity', u'--tags=build:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1', u'--tags=task_name:veyron_minnie-chrome-pfq/R60-9496.0.0-rc1-sanity', u'--tags=board:veyron_minnie', '--', '/usr/local/autotest/site_utils/run_suite.py', '--build', u'veyron_minnie-chrome-pfq/R60-9496.0.0-rc1', '--board', u'veyron_minnie', '--suite_name', u'sanity', '--pool', u'bvt', '--num', '1', '--file_bugs', 'True', '--priority', u'PFQ', '--timeout_mins', '180', '--retry', 'False', '--minimum_duts', '1', '--suite_min_duts', '1', '--offload_failures_only', 'False', '--job_keyvals', "{'datastore_parent_key': ('Build', 1475832, 'BuildStage', 43355589L)}", '--json_dump', '-m', '114037141']


@@@STEP_FAILURE@@@
01:14:06: ERROR: Traceback (most recent call last):
  File "/b/cbuild/repository/chromite/cbuildbot/stages/generic_stages.py", line 628, in Run
    self.PerformStage()
  File "/b/cbuild/repository/chromite/cbuildbot/stages/test_stages.py", line 416, in PerformStage
    ('Build', build_id, 'BuildStage', self._build_stage_id)})
  File "/b/cbuild/repository/chromite/lib/failures_lib.py", line 190, in wrapped_functor
    return functor(*args, **kwargs)
  File "/b/cbuild/repository/chromite/cbuildbot/commands.py", line 1064, in RunHWTestSuite
    json_dump_result = json.loads(s[i:j])
  File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

Looking at the code, I suspect that we just don't get any output that contains the embedded JSON object, which seems consistent with swarming.py printing these diagnostics:

Waiting for results from the following shards: 0
N/A: 35c21201ea00fd10 None