Error "invalid byte sequence for encoding UTF8" on insert into BYTEA column

I am running Postgres 8.4.1 on Windows XP Professional Service Pack 3. My database is UTF8. My program code is written in Python, and to interface to Postgres I use mxODBC 3.0 and the PostgreSQL Unicode driver PSQLODBCW.DLL version 8.01.02.00 dated 31/01/2006.

 

Today for the first time since upgrading to Postgres 8.4.1 I tried out part of the code which inserts some binary data into a table. The insert failed with the error "invalid byte sequence for encoding UTF8". That is odd, because the column into which the insert was made is of type bytea: the data is meant to be binary data, not UTF8-encoded Unicode.

 

When I was using Postgres 8.1.4 the same code worked. My code, the mxODBC code and the driver are all unaltered: only the Postgres version has changed. Is there something I can tweak to get it to work on Postgres 8.4.1?

 

I confirm that the driver option "bytea as LO" is checked. That has always been necessary in the past.