FindControl problem

 Hi friends
I am stuck in a problem. i.e. I am getting data from database and assign them to the dynamically created text boxes. When I tried to access them message Object reference not set to an instance of an object or Specified argument was out of the range of valid values appears. I am using FindControl method.
My text boxes are nested in table cell, table row and table then page. And I am using for loop to access them as I have set those ID accordingly. 
I can't understand why Specified argument was out of the range of valid values message apperas. As I have tested the row index while adding through TableX.Rows.GetRowIndex(tr) method and get the 1,2 and so on. 
While accessing this as 
 
1 for(int i=1; i<= x; i++)
2 {
3 TableRow tr = TableX.Rows[i];
4 TextBox txt = tr.F ...