_customGroupGroupBy not working for member summary in CiviReport

See example at http://forum.civicrm.org/index.php/topic,13188.msg60534.html#msg60534

Adding

protected $_customGroupExtends = array( 'Contact', 'Individual', 'Household', 'Organization' );
protected $_customGroupGroupBy = true;

to CRM/Report/Form/Member/Summary.

gives options to show and group by custom as expected but when trying to group by customGroupGroupBy creates bad SQL missing an aliased table name. Inserting the aliased table name lets the query succeed. See the above forum post for example code.