Workbook Open Event Error

Have some code in the workbook open event, which when I run when the workbook is already open works fine, however when triggered when the workbook is actually opened it fails on the first line.

Code:
Private Sub Workbook_Open()
Dim lngLastRow As Long
Dim ws1 As Worksheet
Dim ws2 As Worksheet
Dim ws3 As Worksheet
Dim X As Long
Dim Y As Long

'Define Worksheet

Set ws1 = Sheets("Sheet1")
Set ws2 = Sheets("Sheet2")
Set ws3 = Sheets("Sheet3")
Sheets("Sheet1").Activate