为什么Http代码500返回，虽然我捕获了异常

虽然我的if-子句在我的fiddler中执行，但我看到http错误500而不是404，为什么会这样?

public class GlobalExceptionHandler : ExceptionHandler
    {
        public override void Handle(ExceptionHandlerContext context)
        {
            if (context.Exception is ResourceNotFoundException)
            {
                context.Request.CreateErrorResponse(HttpStatusCode.NotFound, context.Exception.Message);
            }
            else
            {
                base.Handle(context); 
            }
        }
    }

这是我在fiddler Response:

中得到的结果

HTTP/1.1 500 Internal Server Error
Content-Length: 3945
Content-Type: application/json; charset=utf-8
Server: Microsoft-IIS/7.5
X-Powered-By: ASP.NET
Date: Wed, 08 Jul 2015 08:14:45 GMT
{"Message":"An error has occurred.","ExceptionMessage":"The Resource could not be found!","ExceptionType":"ErpApplicationEndpoints.ResourceNotFoundException","StackTrace":"   at  ... Removed for clarity...