Can anyone see what I'm doing wrong I'm getting an ArgumentNullException: Argument column can't be null ?


Code:
...

DataSet ds = testDSet( ...);
result.Tables.Add( ds.Tables[0].Copy() );
result.Tables.Add( ds.Tables[1].Copy() );
//here's the code that generates the error!
result.Relations.Add( result.Tables[0].Columns["CustomerNumber"] , result.Tables[1].Columns["CustomerNumber"] ); 

...

private static DataSet testDSet(...)
{
  ...
  ds.Tables[0].TableName = "test";
  ...
  ds.Tables.Add("Products");
  ...
  DataSet dsProducts = GetProducts(...)
  foreach( DataRow dr2 in dsProducts.Tables[0].Rows )
  {
	ds.Tables["Products"].ImportRow( dr2 );
  }
return ds;
}