Android : Service intent.getStringExtra() always returns null

I have service with this piece of code:

@Override
public int onStartCommand(Intent intent,int flags, int startId) {
    if ( userId == null ) {
        Log.w("service onStart","trying to get extras");
        try{
            userId = intent.getStringExtra("userId");
        }catch(Exception e){
            e.printStackTrace();
            stopSelf();
        }
    }

    return 0;
}

And I'm calling it like this (From another activity):

@Override
public void onStop(){

    Intent i = new Intent(getBaseContext(), CheckForUpdates.class);
    i.putExtra("userId",""+USER._id);

    Log.w("starting service","with userId:"+i.getStringExtra("userId"));
    this.startService(i);

    super.onStop();
}

But I'm always getting Null Pointer exception in service. Can somebody helps me ?

LOG:

08-08 09:16:06.060: W/service onStart(6253): trying to get extras
08-08 09:16:06.060: W/System.err(6253): java.lang.NullPointerException
08-08 09:16:06.070: W/System.err(6253):     at c com.Hennycz.explopics.CheckForUpdates.onStartCommand(CheckForUpdates.java:54)
08-08 09:16:06.070: W/System.err(6253):     at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2508)
08-08 09:16:06.070: W/System.err(6253):     at android.app.ActivityThread.access$1900(ActivityThread.java:137)
08-08 09:16:06.070: W/System.err(6253):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1307)
08-08 09:16:06.070: W/System.err(6253):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-08 09:16:06.070: W/System.err(6253):     at android.os.Looper.loop(Looper.java:137)
08-08 09:16:06.070: W/System.err(6253):     at android.app.ActivityThread.main(ActivityThread.java:4838)
08-08 09:16:06.070: W/System.err(6253):     at java.lang.reflect.Method.invokeNative(Native Method)
08-08 09:16:06.070: W/System.err(6253):     at java.lang.reflect.Method.invoke(Method.java:511)
08-08 09:16:06.070: W/System.err(6253):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:875)
08-08 09:16:06.070: W/System.err(6253):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:642)
08-08 09:16:06.070: W/System.err(6253):     at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:133)
08-08 09:16:06.070: W/System.err(6253):     at dalvik.system.NativeStart.main(Native Method)