odoo pos checkbox always stores TRUE

I have added a checkbox to the PoS customer page (pos.xml) and whether or not I check it, the value stored in the database is always 'TRUE'. Even if I set value="FALSE".

<div class='client-detail'>
    <span class='label'>Newsletter</span>
    <input class='detail'   type="checkbox"  name='newsletter'  value='FALSE'  t-att-value='partner.newsletter'></input>
</div>