Write value to the Rate (discountrate) field

@NApiVersion

On Sales Orders, I want to write a custom value to the field discountrate. However I keep getting an error "An unexpected error has occurred. Please click here to notify support and provide your contact information." when doing so.

/**
* @NApiVersion 2.0
* @NScriptType UserEventScript
* @NModuleScope SameAccount
*/
define(["N/record"], function (record) {
    function onBeforeSubmit(context) {
        var currentRecord = context.newRecord;
        var mycustomdiscountrate = '-10.00';

        currentRecord.setValue({
        fieldId: 'discountrate',
        value: mycustomdiscountrate,
        });

        return true;
    }
    return {
        beforeSubmit: onBeforeSubmit
    };
});

Using the same code to update other fields works. How do I get this to work with the discountrate field?