Invalid attempt to call MetaData when reader is closed exception

Question

I am using Microsoft Practice Enterprice data to connect a WCF to database(s). Notice that this exception is not thrown always. It's happening inside the while statement. Then if I refresh the page (resubmit the login details, MVC app that communicate with this WCF service), the code execute without any problem. Can someone determine why the IDataRader is closed while the stack execution is in the using statement? My code is:

public class LoginService : ILoginService
{
    public Attendee CheckLoginCredentials(string username, string password, int databaseId)
    {
        Database db = DataBaseConnection.GetDatabaseObject(databaseId);
        Attendee attendee = new Attendee();
        try
        {
            DbCommand dbCommandWrapper = db.GetStoredProcCommand("Check_login_WCF");
            db.AddInParameter(dbCommandWrapper, "@Username", DbType.String, username);
            db.AddInParameter(dbCommandWrapper, "@Password", DbType.String, password);

            using (IDataReader dataReader = db.ExecuteReader(dbCommandWrapper))
            {
                while (dataReader.Read())
                {
                    if (!dataReader.IsDBNull(dataReader.GetOrdinal("PersonId")))
                        attendee.PersonId = dataReader.GetGuid(dataReader.GetOrdinal("PersonId"));


                    if (!dataReader.IsDBNull(dataReader.GetOrdinal("UserName")))
                        attendee.UserName = dataReader.GetString(dataReader.GetOrdinal("UserName"));


                    if (!dataReader.IsDBNull(dataReader.GetOrdinal("IsValid")))
                        attendee.ValidCredentials = dataReader.GetBoolean(dataReader.GetOrdinal("ValidCredentials"));
                }
            }


        }
        catch (Exception ex)
        {
            return null;
        }

        return attendee;
    }
}

DataBaseConnection:

public static Database GetDatabaseObject(int databaseId)
{
    string ConnectionString = string.Empty;
    ConnectionString = EventDatabases.GetConnectionString(databaseId);

    Database mydb = new Microsoft.Practices.EnterpriseLibrary.Data.Sql.SqlDatabase(ConnectionString);
    return mydb;
}