Can't use an undefined value as an ARRAY reference at pt-table-checksum line 2206

Command line : /usr/bin/pt-table-checksum -q
Tool Version: pt-table-checksum 2.1.1
MySQL Version of all servers involved: MySQL 5.1.59

I've hit this odd quirk with pt-table-checksum. It seems to be failing to checksum a particular table for some reason:

05-05T16:31:07 Cannot checksum table rt3.GroupMembers: Can't use an undefined value as an ARRAY reference at /usr/bin/pt-table-checksum line 2206.

mysql> show create table GroupMembers\G
*************************** 1. row ***************************
       Table: GroupMembers
Create Table: CREATE TABLE `GroupMembers` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `GroupId` int(11) NOT NULL DEFAULT '0',
  `MemberId` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `GroupMembers1` (`GroupId`,`MemberId`),
  UNIQUE KEY `SHREDDER_GM1` (`MemberId`,`GroupId`)
) ENGINE=InnoDB AUTO_INCREMENT=944016 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

There doesn't appear to be anything sensitive about the data, so I've attached it to this ticket, if that's of any use.