How do I resolve gcloud gmail-api watch 400 (Bad Request)?

Question

Context:

Following instructions (here) for setting up GMail push notifications; and (here) for setting up a GMail Watch on a Pub/Sub topic.

(Pub/Sub topic and subscription has already been created; API client access to the app has been granted in Google Apps)

Problem:

Executing the following code results in the following (JSON) error:

{
    "error": {
        "errors": [{
            "domain": "global",
            "reason": "failedPrecondition",
            "message": "Bad Request"
        }],
        "code": 400,
        "message": "Bad Request"
    }
}

Resolutions Attempted:

1. Tried including the GMail account as the sub parameter, while creating the credentials service.
2. Recreated Pub/Sub topic & subscription.
3. Added domain to Google Developers Console Project.

Code:

import logging
import httplib2
import json

from apiclient import errors
from apiclient.discovery import build
from oauth2client.client import SignedJwtAssertionCredentials

# Read in service credentials form file
with open('secrets.json') as data_file:
    data = json.load(data_file)

client_email = data["client_email"]
private_key = data["private_key"]

credentials = SignedJwtAssertionCredentials(client_email,
                                            private_key,
                                            'https://mail.google.com/')


def build_gmail_service(credentials):
    # Build a authorized, Gmail service object.
    http = httplib2.Http()
    http = credentials.authorize(http)
    return build('gmail', 'v1', http=http)


def watch_gmail(service, user_id='me'):
    request = {
        'topicName': 'projects/app/topics/gmail',
        'labelIds': ['INBOX'],
        'labelFilterAction': "include"
    }

    try:
        # Make the request.
        response = service.users().watch(userId=user_id,
                                         body=request).execute()

        # Print the results of the request
        if 'error' in response:
            # The API executed, but the script returned an error.
            error = response['error']['details'][0]
            print "Script error! Message: {0}".format(error['errorMessage'])
        else:
            # response successful
            return response
    except errors.HttpError as e:
        # The API encountered a problem before the script started executing.
        logging.error('An error occurred: %s', e.content)


service = build_gmail_service(credentials)
watch_gmail(service, 'user@google_apps_domain.com')