How to send message to multiple recipients?

I'm having some trouble sending a message to multiple addresses using the Gmail API. I've successfully sent a message to only one address, but get the following error when I include multiple comma-separated addresses in the 'To' field:

An error occurred: <HttpError 400 when requesting
https://www.googleapis.com/gmail/v1/users/me/messages/send?alt=json returned "Invalid to header">

I'm using the CreateMessage and SendMessage methods from this Gmail API guide: https://developers.google.com/gmail/api/guides/sending

That guide states that the Gmail API requires messages that are RFC-2822 compliant. I again didn't have much luck using some of these addressing examples in the RFC-2822 guide: https://www.rfc-editor.org/rfc/rfc2822#appendix-A

I'm under the impression that 'mary@x.test, jdoe@example.org, one@y.test' should be a valid string to pass into the 'to' parameter of CreateMessage, but the error that I received from SendMessage leads me to believe otherwise.

Please let me know if you can recreate this problem, or if you have any advice on where I may be making a mistake. Thank you!

Edit: Here is the actual code that yields an error...

def CreateMessage(sender, to, subject, message_text):
    message = MIMEText(message_text)
    message['to'] = to
    message['from'] = sender
    message['subject'] = subject
    return {'raw': base64.urlsafe_b64encode(message.as_string())}

def SendMessage(service, user_id, message):
    try:
        message = (service.users().messages().send(userId=user_id, body=message)
           .execute())
        print 'Message Id: %s' % message['id']
        return message
    except errors.HttpError, error:
        print 'An error occurred: %s' % error

def ComposeEmail():
    # build gmail_service object using oauth credentials...
    to_addr = 'Mary Smith <mary@x.test>, jdoe@example.org, Who? <60one@y.test>'
    from_addr = 'me@address.com'
    message = CreateMessage(from_addr,to_addr,'subject text','message body')
    message = SendMessage(gmail_service,'me',message)

Getting "Invalid to header" when sending with multiple recipients (comma delimited) in a single header was a regression that was fixed on 2014-08-25.