gh-ost aborts on trying to UPDATE, errors on invalid character

v1.0.21

gh-ost with --test-on-replica failed like this:

FATAL Error 1366: Incorrect string value: '\xE1...' for column 'column_name' at row 1; query=

column_name is a latin1 VARCHAR and the value coming in the UPDATE statement contained this problematic character:

So this may be related to the safe SQL mode settings.

The binlogs showed this:

first, a row with the relevant id is INSERTed into the ghost table. At the moment, the data contains no \xE1 character.
eventually, there is incoming UPDATE for the original table that introduces the \xE1 character. gh-ost refuses to handle it and aborts