Failure when using mysql_upgrade if 'root'@'localhost' does not exist

mysql> mysql_upgrade -u*** -p --socket=/tmp/mysql.sock1 --force -s

Enter password:

The --upgrade-system-tables option was used, user tables won't be touched.

Phase 1/7: Checking and upgrading mysql database

Processing databases

mysql

mysql.column_stats OK

mysql.columns_priv OK

...

mysql.transaction_registry OK

Phase 2/7: Installing used storage engines... Skipped

Phase 3/7: Fixing views... Skipped

Phase 4/7: Running 'mysql_fix_privilege_tables'

ERROR 1449 (HY000) at line 832: The user specified as a definer ('root'@'localhost') does not exist

FATAL ERROR: Upgrade failed

MariaDB [sys]> update sys_config set value = 128 where variable = 'statement_truncate_len';

ERROR 1449 (HY000): The user specified as a definer ('root'@'localhost') does not exist