When Guest Checkout Email Not Available

[CI_Fellow]

I have developed Payment Module for Magento 2.0 / 2.1, in these modules working fine with Registered users but when checkout as Guest there comes error "Please enter a customer email."

I have used below code under JS

/**
             * Checkout for guest and registered customer.
             */
            if (!customer.isLoggedIn()) {
                serviceUrl = urlBuilder.createUrl('/guest-carts/:cartId/selected-payment-method', {
                    cartId: quote.getQuoteId()
                });
                payload = {
                    cartId: quote.getQuoteId(),
                    email: quote.guestEmail,
                    method: paymentData,
                    billingAddress: quote.billingAddress()
                };
            } else {
                serviceUrl = urlBuilder.createUrl('/carts/mine/selected-payment-method', {});
                payload = {
                    cartId: quote.getQuoteId(),
                    method: paymentData,
                    billingAddress: quote.billingAddress()
                };
            }

Please guide me to fix this issue,

Expertise are welcome!

Thank you.