Incorrect button labels with "preview page" on single page form

Enabling the new "Preview page" feature causes an issue on forms that are a single page (i.e. no pagebreaks).

All of the "submit" buttons (e.g. Submit, Preview, Previous) appear with the text "Submit".

When viewing the form, the following notice is generated:
Notice: Undefined offset: 1 in webform_client_form() (line 2380 of [...]/webform/webform.module).

Which corresponds to

      elseif ($page_num < $page_count) {
        $form['actions']['next'] = array(
          '#type' => 'submit',
          '#value' => $next_page_labels[$page_num], // line 2380

The $page_num is 1, but the $next_page_labels only has a value for index 0.