"Added column does not sort as the last column" when using new python driver

We've just upgraded one of our python apps from using the old cql library to the new python-driver. When running one particular query, it produces the attached assertion error in Cassandra. The query is:

SELECT buffer, id, type, json FROM events WHERE buffer = %(bid)s AND idkey = %(idkey)s ORDER BY id ASC

Where buffer and idkey are integer primary keys, and id is the clustering key (ordered asc).

This query, with identical parameters, does not cause this error using the old cql python library, or with the cqlsh client..