There is no Proposed data to access!!!???

I used ADO.Net to create two tables - parent and child
table. And, added a relation between two tables. Then, I
setup a column of child table to lookup the value from
parent table. So far, everthing is good.

But, when I modified a context of child table, I got a
Exception - "There is no Proposed data to access."!.

Anyone knows this problem? Have a work around?

Attached is a sample:
-----------------------------------------------------------
static void Main(string[] args)
{
// Create a dataset
DataSet ds = new DataSet();

// Create a parent table, and add into the dataset
DataTable parentDT = new DataTable();
parentDT.Columns.Add("ID", System.Type.GetType
("System.Int32"));

parentDT.Columns.Add("Name", System.Type.GetType
("System.String"));

DataRow newRow = parentDT.NewRow();
newRow["ID"] = 0;
newRow["Name"] = "John";
parentDT.Rows.Add(newRow);

ds.Tables.Add(parentDT);

// Create a child table, adn add into the dataset
DataTable childDT = new DataTable();
childDT.Columns.Add("ID", System.Type.GetType
("System.Int32"));
childDT.Columns.Add("Name", System.Type.GetType
("System.String"));
childDT.Columns.Add("Description", System.Type.GetType
("System.String"));

newRow = childDT.NewRow();
newRow["ID"] = 0;
newRow["Description"] = "Administrator";
childDT.Rows.Add(newRow);

s.Tables.Add(childDT);

// Create a relation
ds.Relations.Add("relation", parentDT.Columns["ID"],
childDT.Columns["ID"], false);

// Setup the expresion for the column "Name" of child
table
childDT.Columns["Name"].Expression = "Parent
(relation).Name";

// Now, modify the column "Description" of child table
try
{
DataRow row = childDT.Rows[0];
row["Description"] = "Engineer";

Console.WriteLine("ID:\t" + row["ID"].ToString());
Console.WriteLine("Name:\t" + row["Name"].ToString());
Console.WriteLine("Description:\t" + row
["Description"].ToString());
}
catch(Exception e)
{
Console.WriteLine("Exception:\t" + e.Message);
}
}