使用属性名构建OrderBy表达式

我试图通过MVC3中的WebGrid控件支持排序，该控件通过sort参数将模型上的属性名称传递到我的操作中。

public class Agent {
    public int Id { get; set; }
    public string Name { get; set; }
}
[HttpGet]
public ActionResult Index(string sort = "Id", string sortdir = "ASC") {
    // Define the parameter that we are going to use in the OrderBy clause.
    var param = Expression.Parameter(typeof(Agent), "agent");
    // Now we'll make our lambda function that returns the
    // property's value by it's name.
    var sortExpression = Expression.Lambda<Func<Agent, object>>(Expression.Property(param, sort), param);
    var agents = entities.OrderBy(sortExpression).ToList();
    var model = new PagedResult<Agent> {
        CurrentPage = 1,
        PageCount = 1,
        PageSize = DefaultPageSize,
        Results = agents,
        RowCount = agents.Count
    };
    return View(model);
}

当我尝试通过Name属性(类型为string)对模型进行排序时，此代码有效。然而，如果我尝试按Id排序，我收到错误Expression of type 'System.Int32' cannot be used for return type 'System.Object'。