Why is my app trying to insert null data into my db?

I'm getting an error message from my C#/ASP.NET app that's telling me that it can't insert null data into a column in the database that doesn't accept nulls. Fair enough. Except my problem is that I can't figure out why it's trying to insert a null value in the first place, since when I query the data that (I think) I'm inserting, it's not null. The code is below. Here's the application trace:

userTable != null
number of rows in userTable = 1
client_id = 100
update user record failed with message: Cannot insert the value NULL into column 'client_id', table 'equickbooks.dbo.Users'; column does not allow nulls. INSERT fails.
The statement has been terminated.
Stack trace:   at System.Data.Common.DbDataAdapter.UpdatedRowStatusErrors(RowUpdatedEventArgs rowUpdatedEvent, BatchCommandInfo[] batchCommands, Int32 commandCount)
  at System.Data.Common.DbDataAdapter.UpdatedRowStatus(RowUpdatedEventArgs rowUpdatedEvent, BatchCommandInfo[] batchCommands, Int32 commandCount)
  at System.Data.Common.DbDataAdapter.Update(DataRow[] dataRows, DataTableMapping tableMapping)
  at System.Data.Common.DbDataAdapter.UpdateFromDataTable(DataTable dataTable, DataTableMapping tableMapping)
  at System.Data.Common.DbDataAdapter.Update(DataTable dataTable)
  at KJI.Components.UsersDB.UpdateUserRecord(DataTable userTable, DataTable rolesTable, DataTable permsTable)
Exception type: System.Data.SqlClient.SqlException

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myConnection = new SqlConnection(ConfigurationSettings.AppSettings["connectionString"]);
myConnection.Open();
txn = myConnection.BeginTransaction();
SqlDataAdapter myDA = new SqlDataAdapter( "select * from users where id=-1", myConnection );
myDA.MissingMappingAction = MissingMappingAction.Ignore;
myDA.SelectCommand.Transaction = txn;
SqlCommandBuilder myCB = new SqlCommandBuilder( myDA );
 
if (userTable != null) 
	{
	HttpContext.Current.Trace.Write("userTable != null");
         HttpContext.Current.Trace.Write("number of rows in userTable = " + userTable.Rows.Count);
	DataRowState state = userTable.Rows[0].RowState;
					HttpContext.Current.Trace.Write("UsersDB.UpdateUserRecord", "client_id = " + userTable.Rows[0]["client_id"]);
         HttpContext.Current.Trace.Write("UsersDB.UpdateUserRecord", "id = " + userTable.Rows[0]["id"]);
	myDA.Update( userTable );
	HttpContext.Current.Trace.Warn("myDA.Update( userTable ) successful"); // not executed