TemplateSyntaxError: could not parse filter argument

TemplateSyntaxError: could not parse filter argument

$ python manage.py runserver
...
TemplateSyntaxError: Could not parse the remainder: 'lower(' from 'username|lower('

Why this happens

Forgot closing parenthesis or quotes around arguments.

Fix

Correct filter usage.

Wrong code

{{ username|default:Guest }}

Fixed code

{{ username|default:"Guest" }}