>Since you're using VS2005, you can utilize the new debugger to find out what's in the dataset. So set a breakpoint after the Fill statement, and put the mouse over the object reference to the dataset. You should see a small magnifying glass - click on that, and VS2005 should display the debugger visualizer for the dataset/datatable. Take a look at the contents.
I think this was mostly related that the sheet contained some empty rows.