pt-online-schema-change error copying rows: Undefined subroutine &pt_online_schema_change::get

pt-online-schema-change --alter "MODIFY <some column> VARCHAR(512) NOT NULL" --execute D=<data base>,t=<table>,u=<user>,p=<password>
percona-toolkit 2.2.2
Server version: 5.5.31-30.3-log Percona Server (GPL), Release 30.3

Undefined subroutine &pt_online_schema_change::get called at /usr/bin/pt-online-schema-change line 10238.

Column changed from text to -> VARCHAR(512)

Operation, tries, wait:
  copy_rows, 10, 0.25
  create_triggers, 10, 1
  drop_triggers, 10, 1
  swap_tables, 10, 1
  update_foreign_keys, 10, 1
Altering `database_name`.`table_name`...
Creating new table...
Created new table database_name.table_name_new OK.
Altering new table...
Altered `database_name`.`_table_name_new` OK.
Creating triggers...
Created triggers OK.
Copying approximately 2538984 rows...
Dropping triggers...
Dropped triggers OK.
Dropping new table...
Dropped new table OK.
`database_name`.`table_name` was not altered.
 (in cleanup) Error copying rows from `database_name`.`table_name` to `database_name`.`_table_name_new`: Undefined subroutine &pt_online_schema_change::get called at /usr/bin/pt-online-schema-change line 10238.

I ultimately resolved this by stopping slave executing command again waited till I got slave_lag message then when I restarted slave it continued successfully ... have tried since unable to reproduce. But there is at least an error in the logic for that handles the error.

PTDEBUG output attached