Drupal 7 で theme.inc で発生する Fatal error の対処方法

Fatal error: Unsupported operand types in /PATH/TO/DRUPAL_ROOT/includes/theme.inc on line 637

function mymodule_theme($existing, $type, $theme, $path) {
  $themes = array(
    'special_element' => array(
      'variables' => array(
        'something_special' => NULL,
      ),
    ),
  );
);

function mymodule_theme($existing, $type, $theme, $path) {
  $themes = array(
    'special_element' => array(
      'variables' => array(
        'something_special' => NULL,
      ),
    ),
  );
  return $themes;
);