Activesheet Returns Runtime Error 91

Private Sub Workbook_Open()
'Application. ScreenUpdating = False ' Turns off screen updating
Dim Filename As String
Dim nFam As Integer
Dim cRow As Integer
Dim i As Integer
ws As Worksheet

Filename = "O:BVRMusers_sharedJCIntraday ToolFamily List.xls"
Workbooks.Open Filename 'Opens Family List

'Count the number of rows (Families) in Filename (Family List)
Set ws = ActiveSheet
With ws
. Cells(1, 1).Select
cRow = .UsedRange.Rows.Count
End With
End Sub

i cannot get ws to be set as the ActiveSheet. i receive a run time error 91 each time and cannot figure out what is wrong. i've used this code before under subform functions, but this is the first time i've placed it in the "ThisWorkbook" area.