there are no lables for preview and submit buttons

I'm trying to modify the privatemsg_new form with form_alter hook from my module to execute aditional functionalities, but i can't change the preview or send buttons because they have no labels.

I solve the problem adding lables to the module, but i don't want to have a modified module, so i ask you to update the module with some lables so anyone can change those buttons from another.

  $form[] = array(
      '#type' => 'submit',
      '#value' => t('Preview')
      );
  $form[] = array(
      '#type' => 'submit',
      '#value' => t('Send private message')
      );

I have changed to:

  $form['prewiew_button'] = array(
      '#type' => 'submit',
      '#value' => t('Preview')
      );
  $form['submit_button'] = array(
      '#type' => 'submit',
      '#value' => t('Send private message')
      );

thank you