Displaying Column Totals in a Footer

I am trying to get the totals for rows and thenshow them in a footer, I keep getting


"Message="Index was out of range. Must be non-negative and less than the size of the collection."
System.ArgumentOutOfRangeException was unhandled by user code
Message="Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index"
ParamName="index"
Source="mscorlib"

for this line Dim TitheCount As Double = (CDbl(GridView1.Rows.Item(6).Cells.ToString()))


Here is my code