Oops when running false for required in a step

I am working on a step in the hadoop-processing spell so users may optionally run a Terasort benchmark.

My step.yaml looks like:
title: Terasort Benchmark
description: |
  Runs teragen to generate sample data, and then runs terasort to sort that data.
viewable: True
required: False

Code at https://github.com/arosales/spells/tree/master/hadoop-processing

When I run this spell I get an error:
"Could not read output from step . . . step-04_run-terasort

Stack trace:
$ conjure-up ~/devel/conjure-up/spells/hadoop-processing
Exception in ev.run():
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/ubuntui/ev.py", line 83, in run
    cls.loop.run()
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 278, in run
    self._run()
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 376, in _run
    self.event_loop.run()
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 1326, in run
    self._loop.run_forever()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 345, in run_forever
    self._run_once()
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1312, in _run_once
    handle._run()
  File "/usr/lib/python3.5/asyncio/events.py", line 125, in _run
    self._callback(*self._args)
  File "/usr/lib/python3/dist-packages/urwid/raw_display.py", line 393, in <lambda>
    event_loop, callback, self.get_available_raw_input())
  File "/usr/lib/python3/dist-packages/urwid/raw_display.py", line 493, in parse_input
    callback(processed, processed_codes)
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 403, in _update
    self.process_input(keys)
  File "/usr/lib/python3/dist-packages/urwid/main_loop.py", line 503, in process_input
    k = self._topmost_widget.keypress(self.screen_size, k)
  File "/usr/lib/python3/dist-packages/urwid/wimp.py", line 643, in keypress
    return self._current_widget.keypress(size, key)
  File "/usr/lib/python3/dist-packages/urwid/container.py", line 1128, in keypress
    return self.body.keypress( (maxcol, remaining), key )
  File "/usr/lib/python3/dist-packages/urwid/decoration.py", line 836, in keypress
    return self._original_widget.keypress((maxcol,), key)
  File "/usr/lib/python3/dist-packages/urwid/container.py", line 1587, in keypress
    key = self.focus.keypress(tsize, key)
  File "/usr/lib/python3/dist-packages/urwid/decoration.py", line 621, in keypress
    return self._original_widget.keypress(maxvals, key)
  File "/usr/lib/python3/dist-packages/urwid/container.py", line 1587, in keypress
    key = self.focus.keypress(tsize, key)
  File "/usr/lib/python3/dist-packages/urwid/wimp.py", line 535, in keypress
    self._emit('click')
  File "/usr/lib/python3/dist-packages/urwid/widget.py", line 463, in _emit
    signals.emit_signal(self, name, self, *args)
  File "/usr/lib/python3/dist-packages/urwid/signals.py", line 264, in emit
    result |= self._call_callback(callback, user_arg, user_args, args)
  File "/usr/lib/python3/dist-packages/urwid/signals.py", line 294, in _call_callback
    return bool(callback(*args_to_pass))
  File "/usr/lib/python3/dist-packages/ubuntui/views/error.py", line 42, in cancel
    raise SystemExit("Install exited because of error.")
SystemExit: Install exited because of error.
Install exited because of error.
arosales@x230:~$