I'm getting an error message when trying to use a variable with async and poll options on a task. It's no problem to use variables elsewhere, but for poll and async, it's a problem. Example below with just poll. Thoughts?

create-ec2-instances.yml

---
- name: Create ec2 instances
  hosts: local
  gather_facts: True
  vars_files:
    - group_vars/all
  vars:
    poll_interval: 5
  roles:
    - { role: create-instances, query_group: 'assignment_take_small' }
    - { role: create-instances, query_group: 'visit_scores' }

main.yml

- name: Create aws instances
  local_action:
    module: ec2
      key_name={{ keypair }}
      group_id={{ security_group }}
      instance_type={{ instance_type }}
      image={{ image }}
      wait=true
      wait_timeout={{ wait_timeout }}
      region={{ region }}
      count={{ query_groups[query_group]['count'] }}
  async: 1800
  poll: '{{ poll_interval }}'
  register: ec2

output

Traceback (most recent call last):
  File "/Users/nathan/code/infrastructure/ansible/bin/ansible-playbook", line 268, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/nathan/code/infrastructure/ansible/bin/ansible-playbook", line 208, in main
    pb.run()
  File "/Users/nathan/code/infrastructure/ansible/lib/ansible/playbook/__init__.py", line 228, in run
    play = Play(self, play_ds, play_basedir)
  File "/Users/nathan/code/infrastructure/ansible/lib/ansible/playbook/play.py", line 123, in __init__
    self._tasks      = self._load_tasks(self._ds.get('tasks', []), load_vars)
  File "/Users/nathan/code/infrastructure/ansible/lib/ansible/playbook/play.py", line 452, in _load_tasks
    results += self._load_tasks(data, mv, default_vars, included_sudo_vars, included_additional_conditions, original_file=include_filename, role_name=new_role)
  File "/Users/nathan/code/infrastructure/ansible/lib/ansible/playbook/play.py", line 454, in _load_tasks
    results.append(Task(self,x,module_vars=task_vars,default_vars=default_vars,additional_conditions=additional_conditions,role_name=role_name))
  File "/Users/nathan/code/infrastructure/ansible/lib/ansible/playbook/task.py", line 187, in __init__
    self.async_poll_interval = int(ds.get('poll', 10))  # default poll = 10 seconds
ValueError: invalid literal for int() with base 10: '{{poll_interval}}'