What is wrong with this code?

[legbreaker]

Can any one please tell why I am getting a Null Pointer Exception for this particular code? And also how I can make it work?

It's a constructor.

public puzzle(int number_of_disks, int pause_time) {
disks = number_of_disks;
time = pause_time;
source = new LinkedStack();
for(int i=disks; i>0; i--){

source.push(i);
}

}


Thanks