Understanding Ansible and Jinja2's default() filter

I have been using Ansible at work for the past month to set up some infrastructure. I wanted to express the following in the setup script: ansible_ssh_user: "{{ lookup('env', 'SSH_USER') }}" but wanted to set the user to ubuntu if SSH_USER was not set. For some reason, this would not work: ansible_ssh_user: "{{ lookup('env', 'SSH_USER') | default('ubuntu') }}" Ansible provides several functions that can extract various string values from the environment.