Theme_button function bug

I found a bug in the theme_button function in form.inc at line 964

i change where puts $element['#button_type'] for $element['#type'], if you don't make that the class is empty like class="form- "

If it isn't a bug sorry for the problems.

function theme_button($element) {
  //Make sure not to overwrite classes
  if (isset($element['#attributes']['class'])) {
    //$element['#attributes']['class'] = 'form-'. $element['#button_type'] .' '. $element['#attributes']['class'];
    $element['#attributes']['class'] = 'form-'. $element['#type'] .' '. $element['#attributes']['class'];
  }
  else {
    //$element['#attributes']['class'] = 'form-'. $element['#button_type'];
    $element['#attributes']['class'] = 'form-'. $element['#type'];
  }

  return '<input type="submit" '. (empty($element['#name']) ? '' : 'name="'. $element['#name'] .'" ') .'value="'. check_plain($element['#value']) .'" '. drupal_attributes($element['#attributes']) ." />\n";
}