I have some custom modules developed from scratch in that Monetory fields are not taking 0 value instead its reverting to previous value.
installation : odoo 13.0 exe installation 

installed on : 13.0.20200412

Example : 
In form view xxx_monetory : 5000 --> 0 when I click on save button the previous value i.e 5000 is reverting back to that field.
This is happening only in custom modules not in standard modules like CRM, Sales etc.
Please help me out on this.

Field declaration :

In .py file

company_id = fields.Many2one('res.company', store=True, copy=False, string="Company", default=lambda self: self.env.user.company_id.id)
currency_id = fields.Many2one('res.currency', string="Currency", related='company_id.currency_id', default=lambda self: self.env.user.company_id.currency_id.id)
fee = fields.Monetary(string="Fee")

In .xml file

name="currency_id" invisible="1"/>
name="fee"/>