Why is Android 8 complaining about Bad notification for startForeground

PLEASE HOLD ON, THE PROBLEM SEEMS TO BE ELSEWHERE . . .

The following code is generating the error on stock android 8.1:

if (Build.VERSION.SDK_INT >= 26) {

    String              channelId  = "com.myapp.notification.channel";
    String              channelName = "Myapp Background Service";
    NotificationChannel channel     = new NotificationChannel(channelId, channelName, NotificationManager.IMPORTANCE_DEFAULT);
//            channel.setLightColor(Color.BLUE);
//            channel.setLockscreenVisibility(Notification.VISIBILITY_PRIVATE);
//            channel.setSound(null, null);  //disable the sound

    NotificationManager manager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
    manager.createNotificationChannel(channel);


    Notification notification = new NotificationCompat.Builder(context, channelId)
            .setOngoing(true)
            .setContentTitle("Myapp")
            .setContentText("Myapp")
            .setSmallIcon(R.drawable.ic_notif_icon)
            .setPriority(PRIORITY_HIGH)
            .setCategory(Notification.CATEGORY_SERVICE)
            .build();

    context.startForeground(14122, notification);
}

The error:

2018-12-14 17:17:39.387 24290-24290/com.myapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.myapp, PID: 24290
    android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=null pri=0 contentView=null vibrate=null sound=null defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1792)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6523)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:865)

the stack-trace reads channel=null ...which hints for not pushing the notification into a channel. my suspicion would be the channel ID com.myapp.notification.channel - while the default is DEFAULT_CHANNEL_ID (with underscores instead of dots). the documentation does not tell how a valid channel ID should look alike, while that's actually the only one difference I can spot there. the examples all use uppercase or lowercase characters, with underscores; the length of the string might also play a role. the channel IDs are even per package, therefore the name of the package in the channel ID is redundant information. this question also seems quite related.