Determining if a worksheet exists

Hi when I try to do the following I get an error 'Subscript out of range'

    If Not .Worksheets(ws_name_string) Is Nothing Then

To do this do I HAVE to then have a worksheet variable to test against, such as:

Dim ws as Worksheet

With ThisWorkbook
    on error resume next
    Set ws = .worksheets(ws_name_string) 
    on error goto 0
    If Not ws Is Nothing Then
    ...
    ...
    End If
End With

I was hoping I might be able to achive the same test within one line?

Thanks