Dependent won't work if #options of dependent item isn't a key array

Took me the better part of the morning to figure this one out:

This code will not work:

function my_module_form(&$form_state) {
  ctools_include('dependent');
  
  $form = array();
    
  $form['payment_method'] = array(
    '#type' => 'select',
    '#title' => t('Type'),
    '#options' => array('Cash', 'Credit'),
    '#default_value' => NULL,
  );
  
  $form['cash_payment'] = array(
    '#type' => 'select',
    '#title' => t('Currency'),
    '#options' => array('UK Pounds', 'Dollars', 'Euro'),
    '#process' => array('ctools_dependent_process'),
    '#dependency' => array('edit-payment-method' => array('Cash')),
  );

  $form['credit_payment'] = array(
    '#type' => 'select',
    '#title' => t('Type of creditcard'),
    '#options' => array('Visa', 'Mastercard', 'American express'),
    '#process' => array('ctools_dependent_process'),
    '#dependency' => array('edit-type' => array('Credit')),
  );
}

The problem is that the #options array of $form['payment_method'] has to be a keyed array.

So: this will work:

function my_module_form(&$form_state) {
  ctools_include('dependent');
  
  $form = array();
    
  $form['payment_method'] = array(
    '#type' => 'select',
    '#title' => t('Type'),
    '#options' => array('cash' => 'Cash', 'credit' => 'Credit'),
    '#default_value' => NULL,
  );
  
  $form['cash_payment'] = array(
    '#type' => 'select',
    '#title' => t('Currency'),
    '#options' => array('UK Pounds', 'Dollars', 'Euro'),
    '#process' => array('ctools_dependent_process'),
    '#dependency' => array('edit-payment-method' => array('cash')),
  );

  $form['credit_payment'] = array(
    '#type' => 'select',
    '#title' => t('Type of creditcard'),
    '#options' => array('Visa', 'Mastercard', 'American express'),
    '#process' => array('ctools_dependent_process'),
    '#dependency' => array('edit-type' => array('credit')),
  );
}

The problem is that #options accepts keyed and non-keyed arrays and parses them fine. If your array is non-keyed, then the Form API will approach it as a numerical keyed array: array(0 => 'Cash', 1 => 'Credit'); So, apart from explicitly providing an associative array to the form item, we could change the #dependency property to this: '#dependency' => array('edit-type' => array(0)), with 0 being the numerical key.

I didn't find this particularity back in the documentation. I erroneously assumed that I had made a mistake with the ID targetting the wrong element and that JS wasn't working as it should.