Error Passing Extra Parameter to Custom Form


I have a simple custom form that works until I try to add a custom parameter for my module called “forms_admin”.

These are the contents of my forms_admin.routing.yml

  path: 'admin/config/system/forms/{arg1}'
    _form: 'Drupalforms_adminFormFormSettings'
    _title: 'Forms Settings'
    _permission: 'access content'

This is a relevant part of my FormSettings.php file (Currently do not have any custom Validation or Submit actions)

public function buildForm(array $form, FormStateInterface $form_state, $arg1 = NULL) {
    $form['title'] = array(
      '#type' => 'textfield',
      '#title' => t('Title'),
      '#required' => TRUE,

    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => t('Submit'),

return $form;

The moment that I add {arg1} to my forms_admin.routing.yml and , $arg1 = NULL to my FormSettings.php file, I am unable to acces the website.

I receive a white page with the error that states:

The website encountered an unexpected error. Please try again later.

After removing arg1 from my two files and going to the Recent Log messages, this is the error I receive:

SymfonyComponentRoutingExceptionMissingMandatoryParametersException: Some mandatory parameters are missing (“arg1”) to generate a URL for route “forms_admin.settings”. in DrupalCoreRoutingUrlGenerator->doGenerate() (line 182 of /var/www/test_site/docroot/core/lib/Drupal/Core/Routing/UrlGenerator.php).

I have tried changing arg1 to other names, not making $arg1 = NULL, etc. and I receive an error every time.

What am I missing?

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