Firebase FCM error JSON_PARSING_ERROR: Unexpected token END OF FILE at position

I am trying to send notifications through firebase's fcm service using php. Here is what I got so far:

$ch      = curl_init();
$payload = [
    'to'   => '/topics/'.ANDROID_TOPIC,
    'notification' => [
        'message' => 1
    ]
];
$headers = [
    'Content-Type: application/json',
    'Content-length: '.sizeof(json_encode($payload)),
    'Authorization: key='.FIREBASE_KEY
];

curl_setopt($ch, CURLOPT_URL, 'https://fcm.googleapis.com/fcm/send');
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($payload));

$result = curl_exec($ch );
curl_close($ch);

return $result;

However, I am getting Unexpected token END OF FILE at position 2 response from firebase.

What do you think is the reason this is happening?