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 X.X.X 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
mysql-server-5.1.73-7.el6.x86_64
# 

I'm running pt-table-checksum and I'm getting following error:

10-05T17:03:10 Skipping chunk X of Y.Z because MySQL chose no index instead of the word_sid_typeindex.

Please advice.