Reading BLOB (LargeBinary) from SQLite via Flask SQLAlchemy returns None

I am trying to read a BLOB (LargeBinary) object from SQLite using Flask-SQLAlchemy, and I'm getting this error: TypeError: must be string or read-only buffer, not None.

My code is as follows:

@app.route('/csv/<int:job_id>', methods=['GET'])
def get_csv(job_id):
    """ Action to retrieve the compressed CSV from the database """
    job = db.session.query(Job).filter_by(id=job_id).first()
    csv = decompress(job.compressed_csv)
    sio = StringIO()
    sio.write(csv)
    sio.seek(0)
    return send_file(sio,
                     attachment_filename=
                     "{}_{}.txt".format(job_id, time.strftime("%Y%m%d%H%M%S")),
                     as_attachment=True)

And my SQLAlchemy model is:

class Job(db.Model):
    """ SQLAlchemy Job Model """
    id = db.Column(db.Integer, primary_key=True)
    list_type_id = db.Column(db.Integer, db.ForeignKey('list_type.id'),
                             nullable=False)
    list_type = db.relationship('ListType',
                                backref=db.backref('jobs', lazy='dynamic'))
    record_count = db.Column(db.Integer, nullable=False)
    status = db.Column(db.Integer, nullable=False)
    sf_job_id = db.Column(db.Integer, nullable=True)
    created_at = db.Column(db.DateTime, nullable=False)
    compressed_csv = db.Column(db.LargeBinary)

    def __init__(self, list_type_id, record_count=0, status=0, sf_job_id=0,
                 csv=None, created_at=datetime.utcnow()):
        self.list_type_id = list_type_id
        self.created_at = created_at
        self.record_count = record_count
        self.status = status
        self.sf_job_id = sf_job_id
        self.compressed_csv = csv

    def __repr__(self):
        return '<Job {}>'.format(self.id)

Here is my schema:

CREATE TABLE alembic_version (
    version_num VARCHAR(32) NOT NULL
);
CREATE TABLE job (
    id INTEGER NOT NULL, 
    list_type_id INTEGER NOT NULL, 
    record_count INTEGER NOT NULL, 
    status INTEGER NOT NULL, 
    sf_job_id INTEGER NOT NULL, 
    created_at DATETIME NOT NULL, 
    compressed_csv BLOB, 
    PRIMARY KEY (id), 
    FOREIGN KEY(list_type_id) REFERENCES list_type (id)
);
CREATE TABLE list_type (
    id INTEGER NOT NULL, 
    name VARCHAR(80) NOT NULL, 
    PRIMARY KEY (id), 
    UNIQUE (name)
);