In an expense record, the unit amount value must never be 0.00. Hence,  I want an error to be raised when the unit amount is 0.00

I added the below code:

    @api.onchange('unit_amount')
    def write(self):
        if self.unit_amount == 0.00:
            raise UserError(_('You are not allowed to create expenses with 0 value'))

The above user error is raised as soon as the Create button is clicked because, unit amount value is by default set as as 0.00

Please provide an alternate suggestion