IndexError in plugins/lookup/subelements.py after upgrade from 1.9 to 2.0

[JK Laiho]

Upgrading from 1.9.5 to 2.0.1.0 without changing the playbook, a task fails that used to work.

The error:

[DEPRECATION WARNING]: Skipping task due to undefined Error, in the future this will be a fatal error.. This feature will be removed in a 

future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.

An exception occurred during task execution. The full traceback is:

Traceback (most recent call last):

  File "/usr/local/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 89, in run

    items = self._get_loop_items()

  File "/usr/local/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 189, in _get_loop_items

    items = self._shared_loader_obj.lookup_loader.get(self._task.loop, loader=self._loader, templar=templar).run(terms=loop_terms, variables=self._job_vars)

  File "/usr/local/lib/python2.7/site-packages/ansible/plugins/lookup/subelements.py", line 37, in run

    terms[0] = listify_lookup_plugin_terms(terms[0], templar=self._templar, loader=self._loader)

IndexError: list index out of range

fatal: [somehostname]: FAILED! => {"failed": true, "msg": "Unexpected failure during module execution.", "stdout": ""}

to retry, use: --limit @deploy.retry

PLAY RECAP *********************************************************************

somehostname                  : ok=4    changed=1    unreachable=0    failed=1 

The task:

- name: copy apt key files from all roles to /tmp

  become: no

  copy: src="{{ item.1.path }}/{{ item.1.filename }}" dest="/tmp/{{ item.1.filename }}"

  when: "'{{ item.0.role }}' in {{ role_names }} and '{{ item.1.pub }}' not in ansible_local.installed_apt_keys.key_list"

  with_subelements:

    - '{{ apt_keys_by_role }}'

    - '{{ keyfiles }}'

The relevant variables:

apt_keys_by_role:

  - role: common

    keyfiles:

      - path: "{{ inventory_dir }}/roles/common/files/apt_keys"

        filename: linode.gpg

        pub: 325A043E

  - role: dev

    keyfiles:

      - path: "{{ inventory_dir }}/roles/dev/files/apt_keys"

        filename: nodesource.gpg.key

        pub: 68576280

Ideas about what I need to do to fix this for 2.0?

Bonus points: how do you use pdb with Ansible's Python modules? I tried sticking import pdb; pdb.set_trace() before the line that produces the IndexError in /usr/local/lib/python2.7/site-packages/ansible/plugins/lookup/subelements.py, but that turns the error into a BdbQuit; the (pdb) prompt is printed but it insta-quits and keeps on going with the task.