Skipping chunk X of Y.Z because MySQL chose no index instead of the word_sid_typeindex

My environment: # cat /etc/redhat-release Red Hat Enterprise Linux Server release 6.8 (Santiago) # uname -a Linux A.B.C 2.6.32-642.4.2.el6.x86_64 #1 SMP Mon Aug 15 02:06:41 EDT 2016 x86_64 x86_64 x86_64 GNU/Linux # rpm -q mysql-server percona-toolkit mysql-server-5.1.73-7.el6.x86_64 percona-toolkit-2.2.19-1.noarch # I'm running pt-table-checksum and I'm getting following error: > Skipping chunk X of Y.Z because MySQL chose no index instead of the > word_sid_typeindex. mysql> DESCRIBE search_index; +-------+------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------------+------+-----+---------+-------+ | word | varchar(50) | NO | MUL | | | | sid | int(10) unsigned | NO | MUL | 0 | | | type | varchar(16) | YES | | NULL | | | score | float | YES | | NULL | | +-------+------------------+------+-----+---------+-------+ 4 rows in set (0.00 sec) mysql> SHOW TABLE STATUS WHERE Name='search_index'\G; *************************** 1. row *************************** Name: search_index Engine: InnoDB Version: 10 Row_format: Compact Rows: 3155432 Avg_row_length: 51 Data_length: 163250176 Max_data_length: 0 Index_length: 415514624 Data_free: 839909376 Auto_increment: NULL Create_time: 2016-09-01 21:14:58 Update_time: NULL Check_time: NULL Collation: utf8_general_ci Checksum: NULL Create_options: Comment: 1 row in set (0.13 sec) ERROR: No query specified mysql> Please advice.