Insert Changed Data From One DataTable Into A Database Table In A Different DataSet

I want to track changes in an application. So I've made a copy of the main table in that application, and am accessing both tables in the application using two separate datasets.

My plan was to use the HasChanges method of the main DataSet to get changed rows and then insert those into the copy table. When I run the code I get an error:

A first chance exception of type 'System.InvalidCastException' occurred in MyApp.exe

Unable to cast object of type 'MainDataTable' to type 'MainAuditDataTable'.

Here's the code:

If Me.DataSet3.HasChanges Then
' 'look for each row that has changes, not new rows
' 'insert that row into the auditing table

[Code].....