无法获取InternalServerException内容

Question

Controller snippet：

public async Task<IHttpActionResult> MyMethod()
{
   try
   {
       //Do Stuff....
   }
   catch(Exception e)
   {
      return HandleException(e);
   }
}

HandleException Snippet ...

public static HttpResponseMessage HandleException(Exception e)
{
    //Bunch of unrelated stuff...
    return new HttpResponseMessage(HttpStatusCode.InternalServerError) 
               { 
                   Content = new StringContent(e.Message)
               };
}

单元测试代码段

public async Task Test_InternalServerError()
{
   Mock<IBusinessObject> boMocked = new Mock<IBusinessObject>();
   Exception mockedException = new Exception("This is a mocked exception");
   boMocked.Setup(x => x.MyMethod()).ThrowsAsync(mockedException);

   MyController controller = new MyController(boMocked.Object);

   IHttpActionResult actionResult = await controller.MyMethod();
   ExceptionResult contentResult = actionResult as ExceptionResult;

   Assert.IsTrue(contentResult.Exception.Message == mockedException.Message);

}

对于上面的单元测试，我遇到的问题是contentResult总是以NULL返回。

动作结果是返回一个ResponseMessageResult类型的对象，其状态代码为500.看起来它不喜欢ExceptionResult的情况，但我发誓几个月前这对我有用。

任何建议表示赞赏。