Job: http://cautotest/afe/#tab_id=view_job&object_id=60551646

Stack trace:
Traceback (most recent call last):
  File "/usr/local/autotest/server/server_job.py", line 646, in run
    self._execute_code(INSTALL_CONTROL_FILE, namespace)
  File "/usr/local/autotest/server/server_job.py", line 1182, in _execute_code
    execfile(code_file, namespace, namespace)
  File "/usr/local/autotest/server/control_segments/install", line 10, in <module>
    job.parallel_simple(install, machines, log=False)
  File "/usr/local/autotest/server/server_job.py", line 574, in parallel_simple
    return_results=return_results)
  File "/usr/local/autotest/server/subcommand.py", line 93, in parallel_simple
    function(arg)
  File "/usr/local/autotest/server/control_segments/install", line 7, in install
    afe_utils.machine_install_and_update_labels(host)
  File "/usr/local/autotest/server/afe_utils.py", line 192, in machine_install_and_update_labels
    image_name, host_attributes = host.machine_install(*args, **dargs)
  File "/usr/local/autotest/server/hosts/testbed.py", line 289, in machine_install
    build_url, _ = host.stage_build_for_install(build)
  File "/usr/local/autotest/server/hosts/adb_host.py", line 1151, in stage_build_for_install
    self.hostname)
  File "/usr/local/autotest/client/common_lib/cros/dev_server.py", line 579, in resolve
    build, cls.get_available_devservers())
  File "/usr/local/autotest/client/common_lib/cros/dev_server.py", line 507, in get_healthy_devserver
    devserver = devservers.pop(hash_index)
AttributeError: 'tuple' object has no attribute 'pop'