Custom ChangeMonitor for .Net MemoryCache causes invalid operation exception

I have written my own custom change monitor class for the .NET MemoryCache. It seems to initialize fine, but when I attempt to add it to the Cache, it throws an InvalidOperation exception - The method has already been invoked, and can only be invoked once.

My change monitor class:

internal class MyChangeMonitor : ChangeMonitor
{
    private Timer _timer;
    private readonly string _uniqueId;
    private readonly TypeAsOf _typeAsOf;
    private readonly string _tableName;

    public GprsChangeMonitor(TypeAsOf typeAsOf, string tableName)
    {
        bool initComplete = false;
        try
        {
            _typeAsOf = typeAsOf;
            _tableName = tableName;

            _uniqueId = Guid.NewGuid().ToString();
            TimeSpan ts = new TimeSpan(0, 0, 5, 0, 0);
            _timer = new Timer {Interval = ts.TotalMilliseconds};
            _timer.Elapsed += CheckForChanges;
            _timer.Enabled = true;
            _timer.Start();
            initComplete = true;
        }
        finally 
        {
            base.InitializationComplete();
            if(!initComplete)
                Dispose(true);
        }
    }

    void CheckForChanges(object sender, System.Timers.ElapsedEventArgs e)
    {
        //check for changes, if different
        base.OnChanged(_typeAsOf);
    }
 }

The code I use to create the cache policy and add the key/value pair to the cache:

CacheItemPolicy policy = new CacheItemPolicy
{
    UpdateCallback = OnCacheEntryUpdateCallback
};

policy.AbsoluteExpiration = SystemTime.Today.AddHours(24);
//monitor the for changes
string tableName = QuickRefreshItems[type];
MyChangeMonitor cm = new MyChangeMonitor(typeAsOf, tableName);
policy.ChangeMonitors.Add(cm);
cm.NotifyOnChanged(OnRefreshQuickLoadCacheItems);

MyCache.Set(cacheKey, value, policy);

The Set call throws the invalid operation exception which is weird because,