ansible variable undefined

####### Playbook provision.yml ###################

- hosts: mw_hosts
  # use strategy: debug when you want to debug your playbook
  strategy: debug

  gather_facts: no

  vars_files:
    - defaults/main.yml
    - vars.yml

  pre_tasks:
    - name: Record hostname if not defined
      set_fact:
        wiki_hostname: "{{ inventory_hostname }}"
        when: wiki_hostname is undefined
    - name: Wait for new host to come up
      local_action: wait_for host={{ ansible_ssh_host }} port=22 state=started delay=0
      tags: [connect]
    - setup:
    - include: tasks/swap.yml
      tags: [whitebox, disk]
    - include: tasks/software.yml
      tags: [whitebox, packages]
    - include: tasks/security.yml
      tags: [whitebox, security]
    - include: tasks/users.yml
      tags: [whitebox, users]

  handlers:
    - include: handlers/site.yml

  # install all the goodness
  roles:
    - { role: qualitybox, tags: ["qualitybox", "mediawiki"] }
    - { role: install_git, tags: ["qualitybox", "mediawiki"] }
    - { role: deploy_mw_host, tags: ["qualitybox", "mediawiki"] }

# Add more plays to the playbook
- include: install_mediawiki.yml
- include: install_extensions.yml
- include: install_skins.yml
# finally create the wiki
#  - include: create_new_wiki.yml
#  - vars: WIKI_NAME=mywiki


######### Which eventually includes roles/install_mediawiki/tasks/main.yml ############
#########                           containing task                        ############

- name: grant access to empty wiki to wiki user (db host != wiki host)
  mysql_user:
    name={{ MYSQL_WIKI_USER }}
    password={{ MYSQL_WIKI_PASSWORD }}
    priv="{{ EMPTY_WIKI_NAME }}.*:SELECT,INSERT,UPDATE,DELETE,INDEX,ALTER,CREATE,DROP,CREATE TEMPORARY TABLES"
    append_privs=true
    host={{ wiki_hostname }}
    login_user=root
  when: inventory_hostname != wiki_hostname


####### Gives this output  ###########################################################

TASK [install_mediawiki : grant access to empty wiki to wiki user (db host != wiki host)] ***
task path: /home/greg/src/ansible-digitalocean/roles/install_mediawiki/tasks/main.yml:36
fatal: [wiki.example.com]: FAILED! => {
    "failed": true,
    "msg": "The conditional check 'inventory_hostname != wiki_hostname' failed. The error was: error while evaluating conditional (inventory_hostname != wiki_hostname): 'wiki_hostname' is undefined\n\nThe error appears to have been in '/home/greg/src/ansible-digitalocean/roles/install_mediawiki/tasks/main.yml': line 36, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n# These steps assume that QualityBox role has already been run to create .my.cnf\n- name: grant access to empty wiki to wiki user (db host != wiki host)\n  ^ here\n"
}

NO MORE HOSTS LEFT *************************************************************
        to retry, use: --limit @provision.retry

PLAY RECAP *********************************************************************
wiki.example.com           : ok=9    changed=3    unreachable=0    failed=1