local variable 'update_task' referenced before assignment

The following traceback was encountered when creating
http://paste.openstack.org/show/491595/

..which failed creation, and then updating with the following template
http://paste.openstack.org/show/491602/

Traceback (most recent call last):
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/service.py", line 126, in log_exceptions
    gt.wait()
  File "/opt/heat/lib/python2.7/site-packages/eventlet/greenthread.py", line 175, in wait
    return self._exit_event.wait()
  File "/opt/heat/lib/python2.7/site-packages/eventlet/event.py", line 125, in wait
    current.throw(*self._exc)
  File "/opt/heat/lib/python2.7/site-packages/eventlet/greenthread.py", line 214, in main
    result = function(*args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/service.py", line 117, in _start_with_trace
    return func(*args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/osprofiler/profiler.py", line 117, in wrapper
    return f(*args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/stack.py", line 86, in handle_exceptions
    {'func': func.__name__, 'msg': errmsg})
  File "/opt/heat/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
    self.force_reraise()
  File "/opt/heat/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
    six.reraise(self.type_, self.value, self.tb)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/stack.py", line 81, in handle_exceptions
    return func(stack, *args, **kwargs)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/stack.py", line 1104, in update
    updater()
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/scheduler.py", line 169, in __call__
    self.start(timeout=timeout)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/scheduler.py", line 194, in start
    self.step()
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/scheduler.py", line 217, in step
    next(self._runner)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/scheduler.py", line 286, in wrapper
    subtask = next(parent)
  File "/opt/heat/lib/python2.7/site-packages/heat/engine/stack.py", line 1397, in update_task
    update_task)
UnboundLocalError: local variable 'update_task' referenced before assignment

I haven't found a way to reproduce it, but it's obvious that if an exception is encountered on the following lines:
https://github.com/openstack/heat/blob/a8bf5cd/heat/engine/stack.py#L1379-L1382

..then the following lines will throw the above exception
https://github.com/openstack/heat/blob/a8bf5cd/heat/engine/stack.py#L1422-L1423