arduino + serialport communication + ruby - ruby

Question

I'm trying to turn on an LED connected to an Arduino from a ruby file, as well as sending a string from that Arduino to my ruby file / terminal. (I'm doing these two things separately, to avoid potential problems).
I'm using a USB port to connect Arduino - Computer, and the serialport gem. I'm working in Linux.
To receive the string on my computer:
I have followed several tutorials that all recommend to run this ruby file:
require 'serialport'
port_str = '/dev/ttyACM0'
baud_rate = 9600
data_bits = 8
stop_bits = 1
parity = SerialPort::NONE
sp = SerialPort.new(port_str, baud_rate, data_bits, stop_bits, parity)
while(true) do
message = sp.gets.chomp
puts message
end
And uploaded this to the Arduino:
uint8_t c;
int i;
void setup() {
Serial.begin(9600); // set baud rate
pinMode(13, OUTPUT); // set pin 13 to be output for LED
digitalWrite(13, LOW); // start with LED off
}
void loop() {
while (Serial.available() > 0) { // check if serial port has data
// writes to computer
Serial.println("Hello world");
}
}
I got this error message: undefined method `chomp' for nil:NilClass (NoMethodError). When getting rid of .chomp, at some point I could get some parts of "Hello world" being printed on my terminal, such as "Hell", and then far later "o w", etc.
Now I don't even get anything anymore.
To turn the led on:
In the ruby file, I replaced the message part by
sp.write('a')
And in the arduino file,
void loop() {
while (Serial.available() > 0) { // check if serial port has data
c = Serial.read(); // read the byte
if (c == 'a'){
digitalWrite(13, HIGH); // turn on LED
delay(500); // wait 500 milliseconds
digitalWrite(13, LOW); // turn off LED
delay(500); // wait 500 milliseconds
}
}
}
The led turns on if I remove the condition (c == 'a'), but obviously I need it if I want the arduino to perform different actions.
I'm new to Serial communication, so I'm not quite sure where the error comes from, since I feel somehow some data seems to be transmitted.