Arduino Serial.println and initNode()

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Hi, I am new to ROS and am currently playing around with Arduino. I came across this problem where after I initNode(), the output in serial monitor turns into backward question marks.

  void setup() {
    Serial.begin(9600);

    //nh.initNode();
    //nh.subscribe(sub);

    //myservo.attach(SERVO_PIN);
  }

    void loop() {
    Serial.println(15);//used for testing println
    nh.spinOnce();
    delay(100);
  }

if I comment nh.initNode() the serial monitor works fine. If I uncomment it, the serial monitor turns into backward question marks. Thanks a lot!