Mailchimp batch subscribe error when using node-mailchimp API v2

I'm using the node-mailchimp wrapper for the MailChimp API v2.
I get the following error when calling 'batch-subscribe' or 'batch-unsubscribe' from a function:
-100 'You must specify a id value'
Here's the function:

function doBatchUnsub(theRequest)
{
    api.call('lists', 'batch-unsubscribe', theRequest, function (error, data)
    {
        if (error)
        {
            errorMessage(error);
        }
        else
        {
            console.log(JSON.stringify(data));
        }
    }
    );
}

It actually works fine if I paste the following JSON into the theRequest parameter:

{
    "apikey" : "my-key-us8",
    "id" : "my-id",
    "batch" : [
        {
            "email" : "example@gmail.com"
        },
        {
            "email" : "example@yahoo.com"
        }
    ],
    "delete_member" : true,
    "send_goodbye" : false,
    "send_notify" : false
}

I only get the error when I pass the JSON into the function from another function. When I do a console.log(theRequest) I can see the exact same text is getting sent to MailChimp. It's confusing.

Any ideas?