Run time error 6 Overflow?

Hello, I have this program that im trying to fix, the problem is it comes up with run time error 6 overflow, it states the problem is with the below. But when i delete the data and try it again it will work without the error, very werid!

 Dim RowIndex As Range, Col1Cell As Long
    For RowIndex = Cells(Rows.Count, Col1).End(xlUp).Row To StartRow + 1 Step -1
        For Each Col1Cell In Range(Cells(StartRow, Col1), Cells(RowIndex - 1, Col1))
            If Col1Cell.Value = Cells(RowIndex, Col1).Value _
            And Cells(Col1Cell.Row, Col2).Value = Cells(RowIndex, Col2).Value Then
                Cells(Col1Cell.Row, Col3).Value = Cells(Col1Cell.Row, Col3).Value + Cells(RowIndex, Col3).Value
                Cells(RowIndex, "A").EntireRow.delete xlShiftUp
                Exit For
            End If
        Next Col1Cell
    Next RowIndex

Any ideas?