Exception thrown when viewing token help

At admin/help, when I click on the 'token' link, i.e. admin/help/token I get the following exceptions:

Original
PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT * FROM {system} WHERE type = 'theme' OR (type = 'module' AND status = 1) ORDER BY weight ASC, name ASC; Array ( ) in system_list() (line 165 of /Library/WebServer/Documents/drupal7/includes/module.inc).

Additional
PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT lid, location FROM {locales_source} WHERE source = :source AND context = :context AND textgroup = 'default'; Array ( [:source] => Loading token browser... [:context] => ) in _locale_parse_js_file() (line 1563 of /Library/WebServer/Documents/drupal7/includes/locale.inc).

( ! ) Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away' in /Library/WebServer/Documents/drupal7/includes/database/database.inc on line 2171

I have tracked it to this line in token_help()

    $output .= '<dd>' . theme('token_tree', array('token_types' => 'all', 'click_insert' => FALSE, 'show_restricted' => TRUE)) . '</dd>';

as when I comment out this I get no errors. Do you have any idea why listing the site tokens via the theme function could cause such an error?

Thanks
Jonathan