Unexpected error when sorting by user

I've got webform installed and running as I want it and ready to hand it over to the site admins.

When viewing the list of submissions, which only has one in it at the moment, and trying to sort by user, the following error occurs:

PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'name' in 'order clause': SELECT ws.sid AS sid FROM {webform_submissions} ws WHERE (nid = :db_condition_placeholder_0) ORDER BY name ASC LIMIT 50 OFFSET 0; Array ( [:db_condition_placeholder_0] => 1 ) in PagerDefault->execute() (line 79 of /var/www/gccc/includes/pager.inc).

Sorting by the other column headings works as expected.