040903 A WebService 's Method ,return Value can't be Object

I pay an Answser $40 for This error . My God. I think I can remeber it for ever .^_^

1. This is OK.

  [WebMethod]
  public DataSet ReturnArr()
  {

      DataSet ds = new DataSet();

       ......

       return ds;
  }

2.Below is NG.

  [WebMethod]
  public Object ReturnArr()
  {

       ......

       return ds;
  }