Why does this test keeps failing?

The test is as follows:

[TestMethod()]
        public void IncrementTestNegative() {

            CancellationTokenSource s = new CancellationTokenSource();
            s.CancelAfter(10);

            ManualResetEventSlim evt = new ManualResetEventSlim();

            bool breached = false;
            Task.Run(() => {
                Thread.Sleep(200);
                breached = true;
            }, s.Token).ContinueWith((t) => {
                evt.Set();
            });


            evt.Wait();
            Assert.IsFalse(breached);
        }

If I provide a value of "0" to CancelAfter, then the test case succeeds. Why does this test keep failing with other values.

This is using MS Test framework within Visual Studio Express