.net-4.0 – Asp.Net 4.0 CacheItemPolicy滑动到期不正确？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System.Runtime.Caching;

using Xunit;

namespace Demo.Caching.Test
{
    class MemoryCacheManagerTest
    {
        [Fact]
        public void Test()
        {
            CacheItemPolicy policy = new CacheItemPolicy();
            policy.SlidingExpiration = TimeSpan.FromSeconds(1);

            MemoryCache.Default.Set("cacheKey4",4,policy);
            Assert.Equal(4,MemoryCache.Default.Get("cacheKey4"));
            System.Threading.Thread.Sleep(600);
            Assert.Equal(4,MemoryCache.Default.Get("cacheKey4"));
            // Here I get error
            // Expected: 4,Actual: (null)

            System.Threading.Thread.Sleep(1000);
            Assert.Null(MemoryCache.Default.Get("cacheKey4"));
        }
    }
}