Detect If Workbook Is Already Open

I searched this site and found the code below to check if a worksheet to be updated is already open. It works fine (gets the desired message) if the workbook is open, but if not open I get "Run-time Error 9, Subscript out of range". I've played with it for a couple hours,

VB:
Dim wBook As Workbook
'Check that Summary worksheet is available for update.
Set wBook = Workbooks("CommittedSummary.xls")
If wBook Is Nothing Then 'Not open
'Do nothing
Else 'It is open
i = MsgBox("The 'CommittedSummary.xls' workbook is already in use and
cannot be updated. Please try again when the workbook Is available", _
vbCritical, "Committed Request Edit")
Goto EndUpdate
End If