Flask-SQLAlchemy unmapped index error

Whenever I try and add to the database, the error sqlalchemy.orm.exc.UnmappedInstanceError is given and I have no idea why. Error Message

This is my code:

class Messages(db.Model):
message = db.Column(db.String(160))
tag = db.Column(db.String(10))
pk = db.Column(db.Integer, primary_key=True)

def __init__(self, pk, message, tag):
    self.pk = pk
    self.message = message
    self.tag = tag


@app.route('/', methods=["GET", "POST"])
def hello_world():
form = Submit()
if form.validate_on_submit():
    message = form.message.data
    tag = form.tag.data
    new_message = [message, tag]
    db.session.add(new_message)
    db.session.commit()
    flash("Sucsessfully posted message: {}".format(message))
    return render_template('index.html', form=form,         messages=Messages.query.all())