[web2py] update() is creating a new item

[Relsi Hur]

Hello,

I have a html form, and a action like this:

def edit_my_table():
    form = SQLFORM(db.my_table)
    if form.process(session=None, formname='edit_').accepted:
        db(db.my_table.id == request.args[0]).update(my_field = 
request.post_vars.my_value)
        response.flash = 'OK'
    elif form.errors:
        response.flash = 'NOK'

Well, it is updating 'my_field' in 'my_db' correctly, but it is creating a 
new item too, with the same value of the update.

Database is sqlite.

What am I doing wrong?