Adding Header via Add-In - Compile error: Type Mismatch

I wrote a macro that basically adds 5 lines at the beginning and puts in a heading (don't want to use headers). The macro worked flawlessly, then i decided to add it to my add-in and now I get the compile error "type mismatch" for the following line

Range("A1").Select

- the "A1" is highlighted in particular.

Here is the entire code:

Rows("1:5").Select
Selection.Insert Shift:=xlDown, CopyOrigin:=xlFormatFromLeftOrAbove
Range("A1").Select
ActiveCell.FormulaR1C1 = "1"
With Selection.Font
.Color = -16776961
.TintAndShade = 0
End With
Selection.Font.Bold = True
Range("A2").Select
ActiveCell.FormulaR1C1 = "2"
Selection.Font.Bold = True
Range("A3").Select
ActiveCell.FormulaR1C1 = "3"
Selection.Font.Bold = True
Range("A4").Select
Selection.NumberFormat = "mm/dd/yy"
ActiveCell.FormulaR1C1 = "4"
With Selection
.HorizontalAlignment = xlLeft
.VerticalAlignment = xlBottom
.WrapText = False
.Orientation = 0
.AddIndent = False
.IndentLevel = 0
.ShrinkToFit = False
.ReadingOrder = xlContext
.MergeCells = False
End With
Selection.Font.Bold = True
Range("A5").Select
ActiveCell.FormulaR1C1 = "5"
Selection.Font.Bold = True
With Selection.Font
.Color = -16776961
.TintAndShade = 0
End With

Any help will be greatly appreciated!