Set Statement Problem

It is a a custom function and here it is. The file name is passed and a Set statement is used to determine if the file is open. If the Set statement works, the file is open and if it doesn't, the file is not open. Basically, TC Rent Roll.xlsm is not being recognized by the Set statement in the function or the sub.

Public Function WorkbookIsOpen(wbname) As Boolean

' Returns TRUE if the workbook is open
    
Dim x As Workbook
On Error Resume Next
Set x = Workbooks(wbname)
If Err = 0 Then
    WorkbookIsOpen = True
Else
    WorkbookIsOpen = False
End If

End Function