How can I make this to stop at the last used row? It goes to the row 65536

Sub FillDownByColumn()
Do Until y = Range("A1").CurrentRegion.Rows.Count - 1
Selection.End(xlDown).Activate
Range(Selection, Selection.End(xlDown).Offset(-1, 0)).Select
Selection.FillDown
y = Selection.Row
Loop
End Sub

Thanks in advance