Admin settings form throws illegal choice error

Hi,

I'm having a go at the very first module that I'm creating. However, I'm running into a bit of an issue that I can't seem to figure out. The code below is part of the code used to display the settings on the administration pages. It all appears to work perfectly: the form is displayed and each checkbox has a unique return value and I can successfully submit the form if all checkboxes are empty.

However, as soon as I select any one or more checkboxes, I get a "An illegal choice has been detected. Please contact the site administrator." error. Why?

  // check to see if imagecache module exists, to unluck more options
  // only needs to be done once, because we can then just count the preset array
  $presets = array();
  if (module_exists('imagecache')) {
    $presets = array_merge(array(t('original')), _imagecodes_get_subarray_elements(imagecache_presets(), 'presetname'));
  } else {
    $presets = array(t('original'));
  }

  $form['fieldset_imagecodes'] = array (
    '#type' => 'fieldset',
    '#title' => 'Image codes',
    '#collapsible' => TRUE,
    '#collapsed' => FALSE
  );	
	
  foreach ($presets as $preset) {
    $form['fieldset_imagecodes']['fieldset_preset_'.$preset] = array (
      '#type' => 'fieldset',
      '#title' => $preset,
      '#collapsible' => TRUE,
      '#collapsed' => TRUE
    );	
		
   // create image codes options checkboxes
    $options['code_'.$preset.'_direct'] = t('Direct link');
    $options['code_'.$preset.'_html'] = t('Wrap the %preset image in HTML tags', array('%preset' => $preset));
    $options['code_'.$preset.'_bbcode'] = t('Wrap the %preset image in BBCode tags', array('%preset' => $preset));
    $options['code_'.$preset.'_html_link_original'] = t('Wrap the %preset image in HTML tags, linking to the original image', array('%preset' => $preset));
    $options['code_'.$preset.'_bbcode_link_original'] = t('Wrap the %preset image in BBCode tags, linking to the original image', array('%preset' => $preset));
    $options['code_'.$preset.'_html_link_node'] = t('Wrap the %preset image in HTML tags, linking to the node', array('%preset' => $preset));
    $options['code_'.$preset.'_bbcode_link_node'] = t('Wrap the %preset image in BBCode tags, linking to the node', array('%preset' => $preset));
    if (count($presets) > 1) {		
      $options['code_'.$preset.'_html_link_original_thumb'] = t('Wrap the thumbnail image in HTML tags, linking to the %preset image', array('%preset' => $preset));
      $options['code_'.$preset.'_bbcode_link_original_thumb'] = t('Wrap the thumbnail image in BBCode tags, linking to the %preset image', array('%preset' => $preset));
    }
		
    $form['fieldset_imagecodes']['fieldset_preset_'.$preset]['preset_selection'] = array(
      '#type' => 'checkboxes',
      '#options' => $options,
      '#default_option' => variable_get('preset_selection', 0)
    );

    unset($options);
  } //foreach ($presets as $preset)

For the sake of completeness, here is the _imagecodes_get_subarray_elements function as well:

  /**
   * Create a one-dimensional array for subarray elements of a two-dimensional array
   */
  function _imagecodes_get_subarray_elements($array_name, $key_name) {
    foreach ($array_name as $row) {
      $return_array[] = $row[$key_name];
    }
    return $return_array;
  }

Thanks in advance.