Google Calendar API v3 'events/watch' Returns Null "Address" in Response Despite No Errors

[Romasa102]

Environment details

• OS: Linux 64bit
• PHP version: 7.4.33
• Package name and version: google-api-php-client

Problem

I'm attempting to receive notifications from Google Calendar using the google_calendar_api_v3/events/watch API (https://developers.google.com/calendar/api/v3/reference/events/watch). Although no errors are returned, the response includes a null "address," and the API ultimately fails to work as expected. I can successfully fetch the calendar and events list with the current authorization setup. The PHP script is part of a backend API for an application, and it will not be accessed directly by users via a browser.

I have already verified that the SSL/TLS certificate is correct, and I changed the redirect URL from one with an underscore to one with a hyphen to avoid potential issues.

Here is the relevant code:

index.php

require_once '../google-api-php-client/vendor/autoload.php';
$this->client = new Google\Client();
$this->client->setScopes(Google_Service_Calendar::CALENDAR);
$this->client->setAuthConfig('../client_secret.json');
$this->client->setAccessType('offline');
$refresh_token = "__REFRESHTOKEN__";
$this->client->fetchAccessTokenWithRefreshToken($refresh_token);
$access_token = $this->client->getAccessToken();
$this->client->setAccessToken($access_token);
$this->service = new Google_Service_Calendar($this->client);
$channel = new Google_Service_Calendar_Channel($this->client);
$channel->setId("1412");
$channel->setType("web_hook");
$channel->setAddress("https://sub-domain.example.com/calendar/push_notifications");
$calendarId = "primary";
$watchResponse = $this->service->events->watch($calendarId, $channel);

ClientSecret.json

{
    "web": {
        "client_id": "__CLIENT_ID__",
        "project_id": "__PROJECT_ID__",
        "auth_uri": "https://accounts.google.com/o/oauth2/auth",
        "token_uri": "https://oauth2.googleapis.com/token",
        "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
        "client_secret": "__CLIENT_SECRET__",
        "redirect_uris": [
            "https://{__PROJECT_ID__}.firebaseapp.com/__/auth/handler",
            "https://sub-domain.example.com/",
            "https://sub-domain.example.com/calendar/push-notifications"
        ],
        "javascript_origins": [
            "http://localhost",
            "http://localhost:5000",
            "https://{__PROJECT_ID__}.firebaseapp.com"
        ]
    }
}

Response.json

{
    "address": null,
    "expiration": "__EXPIRATION__",
    "id": "1412",
    "kind": "api#channel",
    "params": null,
    "payload": null,
    "resourceId": "__RESOUCE_ID__",
    "resourceUri": "https:\/\/www.googleapis.com\/calendar\/v3\/calendars\/primary\/events?alt=json",
    "token": null,
    "type": null
}

What I've tried

Since the initial redirectURL included '_' we changed it to '-'.
Confirmed that the SSL/TLS-certificate is correct.

Thanks!