由逻辑错误引发的SemaphorefullException

我有一个多线程应用程序，其中一个类使用信号量来控制读取&写作。这似乎在大多数时候都很好，但是我得到了一些无法解释的信号异常抛出，表明我试图在不应该释放的时候释放。问题是我已经检查过我的代码了。我似乎找不出导致这种情况发生的逻辑缺陷。

如果有人能看看下面的示例代码，我将非常感激。让我知道我哪里错了，因为在这个阶段，我觉得我开始发疯了。

请注意，"真正的"代码不只是一个线程循环，而是非常接近

class Program
{
    static void Main(string[] args)
    {
        for (int i = 0; i < 100; i++)
        {
            Thread t = new Thread(() => ThreadA());
            t.Start();
        }            
    }
    /// <summary>
    /// Run the methods in the worker class
    /// </summary>
    private static void ThreadA()
    {
        Token token = null;
        WorkClass workClass = new WorkClass();
        try
        {
            workClass.BeginRead(out token, "A");                
        }
        finally
        {
            workClass.EndRead(token);
        }
    }       
}
/// <summary>
///  this class does the actual work
/// </summary>
public class WorkClass
{
    private Semaphore _pool = new Semaphore(2, 2);
    public void BeginRead(out Token token, string s)
    {
        Semaphore sem = null;
        try
        {
            // wait for the main semaphore to signal
            _pool.WaitOne();
            // set local semaphore equal to main
            sem = _pool;
            //simulate work
            Thread.Sleep(100);
        }
        finally
        {
            //return the token with the semaphore
            token = new Token(sem, s);
        }
    }
    public void EndRead(Token token)
    {
        try
        {
            // do some more work
        }
        finally
        {
            // release the semaphore if not null
            if (null != token.signal)
            {
                token.signal.Release();
            }
        }
    }    
}

public class Token
{
   internal readonly Semaphore signal;
   internal readonly string s;
    internal Token(Semaphore _signal, string _s)
    {
        this.s = _s;
        this.signal = _signal;
    }
}

public class WorkClass : IDisposable
using (WorkClass workClass = new WorkClass())