Test for file open from Command Button on userform - not displaying open already message

Hi all

From a userform users can open numerous workbooks from command buttons. I am trying to stop users from opening a second copy if the workbook already open. I found and adapted the following code and it DOES stop a second copy of the workbook being opened but instead of displaying the "File already open" message, it displays the "opening file message. Have I missed something small???
Any pointers appreciated

Function IsOpen(wbName As String) As Boolean
    Dim Wb As Workbook
    On Error Resume Next
    Set Wb = Workbooks(wbName)
    If Err = 0 Then IsOpen = True
End Function
 
Sub TestTKSERVICE()
    Const sFilName As String = "P:\My Documents\Truck Files 2008\TK Service.xlsm"
     
    If IsOpen(sFilName) Then
        MsgBox "Truck Service file is already open !"
        Exit Sub
    Else: MsgBox "Opening Truck Service file"

        ChDir "P:\My Documents\Truck Files 2008"
        Workbooks.Open Filename:=sFilName, Notify:=False
    End If
End Sub