Error while saving the front page meta tags

When saving the front page meta tags, you will get the following error:

warning: Invalid argument supplied for foreach() in /Applications/MAMP/htdocs/dr6/sites/all/modules/nodewords/nodewords.module on line 950.

The error is caused by the following code, which is using an array index that doesn't exist:

function nodewords_admin_frontpage_form_submit($form, &$form_state) {
  _nodewords_set('page', '', $form_state['values']['nodewords']);
  drupal_set_message(t('The configuration options have been saved.'));
}

More exactly, it is caused by the fact $form_state['values']['nodewords']['#tree'] is not set to TRUE.