Why does line 58(notifyAll();) in the following code throw an IllegalMonitorStateException and still produce the correct output?

Question

public class Reader extends Thread{ Calculator c;

public Reader(Calculator cal) {
    c=cal;
}
public void run()
{
    synchronized(c)
    {
        try
        {
            System.out.println(""+Thread.currentThread().getName()+" waiting for calculation");
            c.wait();
        }
        catch(InterruptedException e)
        {

        }
        System.out.println("Total="+c.total);

    }
}

public static void main(String args[])
{
  Calculator C=new Calculator();
  new Reader(C).start();
  new Reader(C).start();
  new Reader(C).start();
  C.start();
}

} class Calculator extends Thread { int total;

    public void run()
    {
        synchronized(this)
        {
            for(int i=0;i<100;i++)
                total+=i;
        }
         notifyAll();//notifyAll();
    }
}