don;t understand control index settings

Question

I have an html table that I build dynamically. The table holds a couple of submit buttons and a list of items from a database with a checkbox before each of them. So the user is allowed to check as many items to be copied over to another table. I was trying to add another table row above what I just described simply to create headers for my html table. I have three columns and I would like the users to understand what they are looking at. But when I add the row I get a 'Specified argument was out of the range of valid values.
Parameter name: index' and it highlights this line of code: if (ctrl.Controls[0].Controls[0] is CheckBox)
So it looks like once I add that table row, it messes up my control numbering?

below is a snippet of my code:

 

Table tblMain = new Table();

tblMain.BorderStyle = BorderStyle.Solid;

tblMain.Width = 750;

TableRow tr;

TableCell td1;

TableCell td2;

TableCell td3;

 

SPSite spSiteContractor = new SPSite(strSecuritySiteUrl);

SPWeb spWebContractor = spSiteContractor.OpenWeb();

///Contractors

SPList spListContractor = spWebContractor.Lists["Daily Contractor"];

SPListItemCollection listItems = spListContractor.Items;

string txtVendor;

string txtName;

//Set table row headers
//THIS IS THE CODE THAT TRIGGERS THE ERROR

tr = new TableRow();
td1 = new TableCell();
td2 = new TableCell();
td3 = new TableCell();
tr.Cells.Add(td1);
 tr.Cells.Add(td2);
tr.Cells.Add(td3);

 td1.Font.Size = 8;
td1.Font.Bold = true;
td1.Text = "Contractor Name";

td2.Font.Size = 8;
td2.Font.Bold = true
td2.Text = "Vendor";

tblMain.Rows.Add(tr);

for (int i = 0; i < listItems.Count;i++ )

{

SPListItem item = listItems[i];

txtVendor = string.Empty;

txtName = string.Empty;

if (item["Vendor"] != null)

{

txtVendor = item["Vendor"].ToString();

}

if (item["Last_x0020_Name"] != null)

{

txtName = item["Last_x0020_Name"].ToString();

}

CheckBox chk = new CheckBox();

contractorInfo = new KeyValuePair<string, string>(txtVendor,txtName);

alllist.Add(chk, contractorInfo);

chk.Text = txtName;

tr = new TableRow();

td1 = new TableCell();

td2 = new TableCell();

td3 = new TableCell();

tr.Cells.Add(td1);

tr.Cells.Add(td2);

tr.Cells.Add(td3);

td1.Controls.Add(chk);

td1.Font.Size = 8;

td2.Text = txtVendor;

td2.VerticalAlign = VerticalAlign.Bottom;

td2.Font.Size = 8;

tblMain.Rows.Add(tr);

}

PlaceHolder1.Controls.Add(tblMain);

tblMain.Dispose();

btnCheckInContractor = new Button();

btnCheckInContractor.CausesValidation = true;

btnCheckInContractor.CssClass = "ms-ButtonHeightWidth";

btnCheckInContractor.Text = "Check-In";

btnCheckInContractor.Click += new EventHandler(btnCheckInContractor_Click);

PlaceHolder1.Controls.Add(btnCheckInContractor);