Why does synchronized notifyAll result in a IllegalMonitorStateException?

public class test {
    static Integer foo = new Integer(1);
    public static void main(String[] args) {
        synchronized(foo) {
            foo++;
            foo.notifyAll();
        }
        System.err.println("Success");
    }
}

Exception in thread "main" java.lang.IllegalMonitorStateException
        at java.lang.Object.notifyAll(Native Method)
        at test.main(test.java:6)

static Integer foo = new Integer(1);

public static void main(String[] args) {
    synchronized (foo) {
        foo.notifyAll();
    }
    System.out.println("Success");
}

import java.util.concurrent.atomic.AtomicInteger;

public class SynchronizeOnAPrimitive {
    static AtomicInteger foo = new AtomicInteger(1);
    public static void main(String[] args) {
        synchronized (foo) {
            foo.incrementAndGet();
            foo.notifyAll();
        }
        System.out.println("foo is: " + foo);
    }
}

foo is: 2