NULLReferenceException from SqlDataReader.GetValue()

I am getting a random exception coming out of ADO.NET's SqlDataReader that I can't seem to track down for the life of me. I have put SQL Server and IIS under heavy load, with neither giving me the exceptions except occasionally (they come when there is no load as well). Web Site is built in VS2005 using .NET 2.0.

Basically, I have a web page that caches a few tables from a SQL database (about 10 tables are loaded into cache from this page) and the error can come from any of the tables while they are being read from the database randomly. I had assumed it was possible that a timeout was occuring while reading the data but from what the brief results google shows, there would be a different exception thrown it a timeout had occured.

Here is the exception that is showing up:

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Data.SqlClient.SqlDataReader.ReadColumnHeader(Int32 i)
at System.Data.SqlClient.SqlDataReader.ReadColumn(Int32 i, Boolean setTimeout)
at System.Data.SqlClient.SqlDataReader.GetValueInternal(Int32 i)
at System.Data.SqlClient.SqlDataReader.GetValue(Int32 i)
at Library.Data.DataProxy.ExecuteStoredProcedure(ArrayList& returnValues, TypeFactory factory)

I got the following exception once from the same section of code as well:

System.InvalidOperationException: Invalid attempt to FieldCount when reader is closed.
at System.Data.SqlClient.SqlDataReader.get_FieldCount()
at Library.Data.DataProxy.ExecuteStoredProcedure(ArrayList& returnValues, TypeFactory factory)

The code generating these errors is on line 29 below:

1 /// <summary>
2 /// Executes a stored procedure that returns values
3 /// </summary>
4 /// <param name="returnValues">An arraylist of Types</param>
5 /// <param name="factory">The Type specific factory</param>
6 /// <returns>True if successful, false otherwise</returns>
7 public bool ExecuteStoredProcedure(out ArrayList returnValues, TypeFactory factory)
8 {
9 bool bResult =true;
10 SqlDataReader reader =null;
11
12 returnValues =new ArrayList();
13 MenseType nextType =null;
14
15 try
16 {
17 // Open database connection
18 OpenConnection();
19
20 // execute SP
21 reader = m_sqlCommand.ExecuteReader();
22
23 while(reader.Read())
24 {
25 nextType = factory.CreateInstance();
26
27 for(int i = 0; i < reader.FieldCount; i++)
28 {
29 object val = reader.GetValue(i);
30
31 if(DBNull.Value == val)
32 val =null;
33
34 nextType.SetValue(reader.GetName(i), val);
35 }
36
37 returnValues.Add(nextType);
38
39 }// while
40
41 }// @. try
42 catch(Exception ex)
43 {
44 returnValues =null;
45 m_LastError = ex.ToString();
46 bResult =false;
47
48 #if(DEBUG)
49 Library.IO.Logging.AddSQLMessage(ex);
50 #endif
51
52 }// @. catch
53 finally
54 {
55 // close the reader if nessessary
56 if (null != reader)
57 reader.Close();
58
59 // close db connection
60 CloseConnection();
61
62 }// @. finally
63
64 return bResult;
65
66 }// ExecuteStoredProcedure()

Any input would be greatly appreciated.

Do you need to read every table as an untyped arraylist, why not read each column of each table as a specific data type into a typed structure?