ApplePay completeMerchantValidation fails

Question

We have a site example.com behind ssl that runs a page with ApplePay. We've got a server side that returns a Merchant Session that looks like the following:

{"epochTimestamp":1581975586106,"expiresAt":1581979186106,"merchantSessionIdentifier":"SSH8E666B0...","nonce":"1239e567","merchantIdentifier":"...8557220BAF491419A...","domainName":"example.com","displayName":"ApplePay","signature":"...20101310f300d06096086480165030402010500308..."}

We receive this response in session.onvalidatemerchant as a string and convert it to a Json Object and pass to session.completeMerchantValidation. As a result we get the following error:

Code: "InvalidAccessError"
Message: "The object does not support the operation or argument"

We run the following code on our page:

.....
session.onvalidatemerchant = (event) => {
        const validationURL = event.validationURL;        
        getApplePaySession(validationURL).then(function (response) {
            try {
                let resp = JSON.parse(response);
                session.completeMerchantValidation(resp);
            } catch (e) {
                console.error(JSON.stringify(e));
            }
        });
    };
....

Additional questions:

1. Is the object described above a "correct" Merchant Session opaque that needs to be passed to completeMerchantValidation or it's missing some fields?
2. Is this object needs to be passed as is or it needs to be base64 encoded?
3. Does it need to be wrapped into another object?

Any help or lead is greatly appreciated.