请问下面这段错在哪呀?
string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0; " +
    "Ole DB Services=-4; " +
    "Data Source=e:\\aa\\a.mdb";
string strCmd = "select sum(BookNumber) from SellTable";
OleDbConnection objConn = new OleDbConnection(strConn);
OleDbCommand objCmd  = new OleDbCommand(strCmd,objConn);
string Result ;
objConn.Open();
Result = objCmd.ExecuteScalar().ToString();
objConn.Close();
TxtResult.Text = Result;