Ansible uses the Jinja2 template engine to handle variables. This includes the default filter, which sets a default value if a referenced variable is not explicitly defined somewhere else.
With Ansible it might happen that instead of a skalar variable a key-value is needed, a dictionary. If you just paste the plain text in there, you might run into trouble:
fatal: [test.example.com]: FAILED! => {"changed": false, "msg": "argument env is of type and we were unable to convert to dict: dictionary requested, could not parse JSON or key=value"}
The key-value pair needs to be properly formatted:
"{{ my_variable|default({'key':'value'}) }}"
Thanks to @bcoca for his post about this.