Ansible in dev environment: failing on new deployment (initial run) when trying to deploy Munin to webapp1

I'm trying to set up a new clean development environment using Ansible, and I'm running into the following. The deployment succeeds up until the point that it tries to set up Munin on @webapp1@:

<pre>
PLAY [Configure webapp1 as Munin master for development] **********************

GATHERING FACTS ***************************************************************
ok: [webapp1]

TASK: [munin_master | Ensure existence of munin package] **********************
changed: [webapp1]

TASK: [munin_master | Ensure state of munin master configuration file] ********
fatal: [webapp1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'munin_notification_email_recipient' is undefined", 'failed': True}
fatal: [webapp1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'munin_notification_email_recipient' is undefined", 'failed': True}

FATAL: all hosts have already failed – aborting
</pre>

Despite the error message, I have set @munin_notification_email_recipient@ in @ansible/vars/development.yml@.

However, setting the @munin_notification_email_recipient@ in @ansible/group_vars/all.yml@ fixed this and allowed the deployment to proceed.

Should declaring this variable in one of the environment config files in @ansible/vars@ be sufficient? This is where things are set in our AWS configuration.