Unexpected IllegalStateMonitorException

Question

Sometimes the below code throws IllegalStateMonitorException and sometimes it runs nicely. Not able to understand the scenarios how could it happen as it is always called under lock.lock().

Full Code:

public class OddEvenPrinter implements Runnable {
    private Lock lock = new ReentrantLock();
    Condition conditionFlag = lock.newCondition();
    private int counter = 0;

    public void run(){
        for(int i = 0; i<1000;i++){
            printEven();
            printOdd();
        }
    }

    private void printEven() {
        try{
            lock.lock();
            while(counter%2!=0){
                wait();
            }
            System.out.println(Thread.currentThread().getName()+" thread printing Even :"+counter);
            counter++;
            conditionFlag.signalAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally{
            lock.unlock();
        }
    }

    private void printOdd(){
        try{
            lock.lock();
            while(counter%2==0){
                wait();
            }
            System.out.println(Thread.currentThread().getName()+" thread printing Odd :"+counter);
            counter++;
            conditionFlag.signalAll();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }finally{
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        OddEvenPrinter counter = new OddEvenPrinter();
        Thread t1 = new Thread(counter);
        Thread t2 = new Thread(counter);
        t1.setName("First");
        t2.setName("Second");

        t1.start();
        t2.start();

        t1.join();
        t2.join();
    }
}