Error on pt-table-sync

Synchronizing two large table that contains BLOB data I get this error “Use of uninitialized value in string ne at /usr/bin/pt-table-sync line 5153”.
It’s a bug? With other table it works fine

Can you send the full pt-table-checksum and pt-table-sync commands as well as the toolkit version. Thanks!

[]mysql version mysql-5.6.28-linux-glibc2.5-i686
]pt-table-sync 2.2.16
[*]the two databases resides on different hosts but have the same version
I made a script to lunch this command:
h=conlabfe2,u=checksum_user,p=PWD,P=3308,S=/var/run/mysql/mysql_a.sock &>sync_dati_$1.log &