C# datatable can't find first column

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C#

I have a problem referencing a datatable field. I set up the datatable and fill it with data like this:-

public partial class Form1 : Form
 {
    .........
     public DataTable dtStaff = new DataTable("StaffList");
     DataColumn dc400 = new DataColumn();
     DataColumn dc401 = new DataColumn();
     ........
 public Form1()
     {
         InitializeComponent();
     }
     private void Form1_Load(object sender, EventArgs e)
     {
         dtStaff.Columns.Clear();
         dtStaff.Columns.Add(dc400);
         dtStaff.Columns.Add(dc401);
         dc400.ColumnName = "StaffID"; dc400.Caption = "Person ID"; dc400.DataType = System.Type.GetType("System.Int32");
         dc401.ColumnName = "FullName"; dc401.Caption = "Name"; dc401.DataType = System.Type.GetType("System.String");
         .....
         string SelectString = "SELECT PersonID, FullName FROM People INNER JOIN [Workbench].[dbo].[TimesheetGroups] ON People.TimesheetGroupID=TimeSheetGroups.GroupID";
         SelectString += " WHERE (TimeSheetGroups.Approver1 = " + WBID + " OR TimeSheetGroups.Approver2 = " + WBID + ")";
         using (SqlConnection con7 = new SqlConnection(WorkbenchConnection))
         {
             SqlCommand command07 = new SqlCommand(SelectString, con7);
             try
             {
                 con7.Open();
                 SqlDataAdapter da1 = new SqlDataAdapter(command07);
                 dtStaff.Clear();
                 da1.Fill(dtStaff);
                 con7.Close();
                 da1.Dispose();
             }
             catch
             {
                 ............
             }
         }
         ...........
     }

I know the data fill works because in a Watch window I can see expected results for dtStaff.Rows[0]["FullName"] with various row numbers.
However dtStaff.Rows[0]["StaffID"] gives "... threw an exception of type 'System.ArgumentException'
Attempting to use it elsewhere gives an error 'Column 'StaffID' does not belong to table 'StaffList'
Is there something special about the first column of a Datatable?

What I have tried:

I've tried renaming the field, debugging other places in the code where a datatable works, etc.