Comment 1 for bug 1576036

Test case:

initial dump file attached.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show indexes from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | a | A | 106541 | NULL | NULL | | BTREE | | |
| t1 | 1 | b | 1 | b | A | 106541 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> update t1 set b=null limit 20000;
Query OK, 20000 rows affected, 1 warning (0.51 sec)
Rows matched: 20000 Changed: 20000 Warnings: 1

mysql> show indexes from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | a | A | 95961 | NULL | NULL | | BTREE | | |
| t1 | 1 | b | 1 | b | A | 95961 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> quit
Bye
[user@sandbox ~]$ pt-table-checksum --databases=test --where "1=1" h=127.0.0.1,u=root,p=msandbox,P=21690
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
 LANGUAGE = (unset),
 LC_ALL = (unset),
 LC_CTYPE = "UTF-8",
 LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
04-26T14:41:21 Error checksumming table test.t1: Use of uninitialized value in string ne at /usr/bin/pt-table-checksum line 6768.

            TS ERRORS DIFFS ROWS CHUNKS SKIPPED TIME TABLE
04-26T14:41:21 1 0 0 1 0 0.024 test.t1