Could someone please tell me why the code below, is making my Macro paste the formula down 65536 rows, instead of stopping at the end of the data?? Thank you in advance!!

Range("M2").Select
ActiveCell.FormulaR1C1 = "=SUM(RC[-4]+RC[-3])"

Dim myLastRow As Long
myLastRow = ActiveSheet.UsedRange.Rows.Count
If myLastRow > 1 Then Range("M2").AutoFill Destination:=Range("M2:M" & myLastRow)