Password Protect on Page returns Undefined Index: type

I have the following in Front-End Control Panel YAML to protect a webpage.

protect:
  type: password
  allowed:
    - Password
  form_url: /password-entry

However, instead of presenting the "Password Checkpoint" page on attempted entry, the webpage fails to load with the following error.

in ProtectAPI.php line 40
at HandleExceptions->handleError('8', 'Undefined index: type',
'/public_html/statamic/bundles/Protect/ProtectAPI.php', '40',
array('data' => object(LocalizedData), 'url' => '/material/course-1', 'config'
=> array('password' => array('form_url' => '/password-entry')), 'type' => null,
'protect' => array('protect' => array('allowed' => array('Password'))))) in
ProtectAPI.php line 40

For context, the website runs on Statamic V2 and was working with this configuration up until recently. I cannot trace it back to a particular update, but I suspect it might have something to do with my syntax.

Any help will be greatly appreciated!