Due serial port being sensitive?

Hardware Arduino Due
5

So I just tested again with the code below. I connect the USB cable to programming port of the Due and the other end to my laptop.

As soon as I have a wire coming from pin 19 (RX1) and use my finger touching the wire, the same thing happened immediately.

Weird thing is it does not happen if I use an external power source. I tried printing out the character a but apparently it's empty? Switching to type int for a, I got number 0.

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial1.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial1.available())  {
    char a = Serial1.read();
    Serial.println("I'm not supposed to receive anything!!!");
  }
}