Unable to run the wireless test manually with plainbox, "flsuh" is not the keyword for got_program_output function

CID: 200912-4896

When I tried to run the following command on a SRU system:
plainbox run -i "2013.com.canonical.certification::wireless/wireless_connection_open_bg"

it failed with:
--------------[ Running job 4 / 4. Estimated time left: 0:00:30 ]---------------
--------------------[ wireless/wireless_connection_open_bg ]--------------------
ID: 2013.com.canonical.certification::wireless/wireless_connection_open_bg
Category: 2013.com.canonical.plainbox::uncategorised
... 8< -------------------------------------------------------------------------
Exception in thread queue_worker:
Traceback (most recent call last):
  File "/usr/lib/python3.2/threading.py", line 740, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.2/threading.py", line 693, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3/dist-packages/plainbox/vendor/extcmd/__init__.py", line 625, in _drain_queue
    self._delegate.on_line(*args)
  File "/usr/lib/python3/dist-packages/plainbox/vendor/extcmd/__init__.py", line 663, in on_line
    delegate.on_line(stream_name, line)
  File "/usr/lib/python3/dist-packages/plainbox/impl/runner.py", line 251, in on_line
    self.ui.got_program_output(stream_name, line)
  File "/usr/lib/python3/dist-packages/plainbox/impl/commands/inv_run.py", line 203, in got_program_output
    end='', flush=True, file=stream)
TypeError: 'flush' is an invalid keyword argument for this function

------------------------------------------------------------------------- >8 ---
Outcome: job failed

Debugging information:
ubuntu@200912-4896:~/.cache/plainbox/sessions/pbox-yhaw0f.session$ checkbox check-config
Configuration files:
 - /etc/xdg/plainbox.conf
 - /etc/xdg/checkbox.conf (not present)
 - /home/ubuntu/.config/plainbox.conf (not present)
 - /home/ubuntu/.config/checkbox.conf (not present)
Variables:
   [FEATURE-FLAGS]
   extcmd=legacy
   [submission]
   submit_to_c3=unset
   [submission]
   submit_to_hexr=unset
   [sru]
   email_address=unset
   [sru]
   c3_url=https://certification.canonical.com/submissions/submit/
   [sru]
   lp_url=https://launchpad.net/+hwdb/+submit
   [sru]
   whitelist=/usr/share/2013.com.canonical.certification:checkbox/whitelists/sru.whitelist
Sections:
   [environment]
   BTDEVADDR=00:02:72:3E:83:D6
   OPEN_AC_SSID=ubuntu-cert-ac-open
   OPEN_BG_SSID=ubuntu-cert-bg-open
   OPEN_N_SSID=ubuntu-cert-n-open
   ROUTERS=multiple
   TRANSFER_SERVER=cdimage.ubuntu.com
   WPA_AC_PSK=insecure
   WPA_AC_SSID=ubuntu-cert-ac-wpa
   WPA_BG_PSK=insecure
   WPA_BG_SSID=ubuntu-cert-bg-wpa
   WPA_N_PSK=insecure
   WPA_N_SSID=ubuntu-cert-n-wpa
No validation problems found

root@200912-4896:~$ apt-cache policy checkbox
checkbox:
  Installed: 0.17.10+bzr2832+pkg77+legacy1~ubuntu12.04.1
  Candidate: 0.17.10+bzr2832+pkg77+legacy1~ubuntu12.04.1
  Version table:
 *** 0.17.10+bzr2832+pkg77+legacy1~ubuntu12.04.1 0
        500 http://ppa.launchpad.net/checkbox-dev/ppa/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status
     0.13.10 0
        500 http://us.archive.ubuntu.com/ubuntu/ precise-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu/ precise-updates/main amd64 Packages
     0.13.7 0
        500 http://us.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
root@200912-4896:~$ apt-cache policy plainbox
plainbox:
  Installed: 0.22+bzr3866+pkg31~ubuntu12.04.1
  Candidate: 0.22+bzr3866+pkg31~ubuntu12.04.1
  Version table:
 *** 0.22+bzr3866+pkg31~ubuntu12.04.1 0
        500 http://ppa.launchpad.net/checkbox-dev/ppa/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status
root@200912-4896:~$ plainbox dev list provider
provider '2013.com.canonical.plainbox:manifest'
provider '2013.com.canonical.plainbox:exporters'
provider '2013.com.canonical.plainbox:categories'
provider '2013.com.canonical.certification:certification-client'
provider '2013.com.canonical.certification:checkbox'
provider '2013.com.canonical.certification:plainbox-resources'