SqlDataReader throws NullReferenceException! what could cause this and how can I debug?

1) Exception Information
*********************************************
Exception Type: System.NullReferenceException
Message: Object reference not set to an instance of an object.
Data: System.Collections.ListDictionaryInternal
TargetSite: Void Bind(System.Data.SqlClient.TdsParserStateObject)
HelpLink: NULL
Source: System.Data

StackTrace Information
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   at System.Data.SqlClient.SqlDataReader.Bind(TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult esult)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   at System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet)
   at MyCode.Shared.Data.DataSocket.GetTable(String SPString)
   at <rest of stack trace>

public DataTable GetTable(string SPString)
{
    //Setup the data objects by calling helper
    prepareDataAdaptor(SPString,CommandType.Text);

    dataAdaptor.Fill(dataSet);
    dataAdaptor.SelectCommand.Connection.Close();
    DataTable dt;
    //ensure we dispose okay
    using(dataSet)
    {

        if(dataSet.Tables.Count==0)
        {
            dataSet.Tables.Add(new DataTable("EmptyTable"));
        }

        dt=dataSet.Tables[0];
        //because we are disposing we need to remove the table from the dataset
        dataSet.Tables.Clear();

    }
    return dt;
}

private void prepareDataAdaptor(string SPString,CommandType Type)
{
    checkForConnection();
    dataSet=new DataSet();
    dbCommand.CommandText=SPString;
    dbCommand.CommandTimeout = MySettings.CommandTimeout;
    dataAdaptor.SelectCommand=dbCommand;
    dataAdaptor.SelectCommand.CommandType=Type;
    dataAdaptor.SelectCommand.Connection=dbConnection;
    dataAdaptor.SelectCommand.Parameters.Clear();
}

dataAdaptor.SelectCommand.Connection.Close();