Program crashes when I use Serial.println()?
You should have provided your full Arduino code. Without setup() part of Arduino code, we can not exactly tell what might be wrong. Please provide it and I'll revise my answer as well. 

Check if you have this code in your setup() function:

Serial.begin(9600);


9600 is baud rate and it could be any other supported baud rate(you can see those by opening a Serial monitor and clicking on 9600 baud rate on bottom right edge). 

Related to to comments above, from experience and it can be easily "counted", RAM shouldn't be a problem in such short code.