我得到下面代码的这个错误

First thread about to sleep
thread 1  run
Boolean assignment done.
Woke up and about to invoke wait()
Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at IncorrectSynchronization$1.run(HelloWorld.java:23)
    at java.lang.Thread.run(Thread.java:748)

当线程t1处于休眠状态时，我将另一个线程的锁修改为false。然后抛出这个非法的MonitorStateException。它仍然是同一个对象，为什么修改值会导致illegalmonitorstateexception？
当我从同步块中的另一个线程将锁修改为false时，我不再得到该错误。有人能解释引擎盖下发生的事情的原因吗？

public class HelloWorld{
   public static void main( String args[] ) throws InterruptedException {
        SampleTest.runExample();
    }
}
class SampleTest{
    Boolean flag = new Boolean(true);
    public void example() throws InterruptedException {
        Thread t0 = new Thread(new Runnable() {
            public void run() {
                synchronized (flag) {
                    try {
                        while (flag) {
                            System.out.println("First thread about to sleep");
                            Thread.sleep(2000);
                            System.out.println("Woke up and about to invoke wait()");
                            flag.wait();
                            System.out.println("wait() called");
                        }
                    } catch (InterruptedException ie) {
                    }
                }
            }
        });
        Thread t1 = new Thread(new Runnable() {
            public void run() {
                System.out.println("thread 1  run");
                flag = false;
              }
        });
        t0.start();
        Thread.sleep(200);
        t1.start();
        t0.join();
        t1.join();
    }
    public static void runExample() throws InterruptedException {
        SampleTest test = new SampleTest();
        test.example();
    }
}