Trouble With SqlDataReader

hi..i am kind of new to asp.net n having trouble with the SqlException error message.. n this code works as 1st page pass the id to second page and  the second page took the id to execute the query..i dun know the wer the error occurs..can give a help..Thanks.

private void Page_Load(object sender, System.EventArgs e)

{



SqlConnection connection = null;

SqlCommand command = null;

string sqlConnection = null;

string sql = null;

string ab = null;

sqlConnection = ConfigurationSettings.AppSettings["MSQLConnectionString"];

connection = new SqlConnection(sqlConnection);

if (!Page.IsPostBack)

{

try

{

if (Request.QueryString["categoryID"] == null)

{

}

else

{

ab= Request.QueryString["categoryID"].ToString(); //getting the id from page that pass this values





sql = "Select groupname, categoryid, description from groups where groups.categoryid=?"; // can this query execute?

command = new SqlCommand(sql, connection);

connection.Open();

command.Parameters.Add(new SqlParameter("categoryid", ab));

reader = command.ExecuteReader();  // error on here "SqlException"

while (reader.Read())

{

group.InnerText = reader["groupname"].ToString();

desc.InnerText = reader["description"].ToString();





}

}

}

finally

{

if (reader != null)

{

reader.Close();

}



if (connection != null)

{

connection.Close();

}

}

}