Database Query Error

I get the following database syntax error when deleting a submitted webform entry from the results area.

warning: pg_query() [function.pg-query]: Query failed: ERROR: syntax error at or near ")" LINE 1: ...u ON u.uid = s.uid WHERE sd.nid = 6 AND s.sid IN () ORDER BY... ^ in /drupal_base_directory/includes/database.pgsql.inc on line 139.
user warning: query: SELECT s.*, sd.cid, sd.no, sd.data, u.name, u.mail, u.status FROM webform_submissions s LEFT JOIN webform_submitted_data sd ON sd.sid = s.sid LEFT JOIN users u ON u.uid = s.uid WHERE sd.nid = 6 AND s.sid IN () ORDER BY sid ASC, cid ASC, no ASC in /drupal_base_directory/sites/all/modules/webform/webform_submissions.inc on line 168.