I need help with this macro. It is not successfully creating the custom property. Can anyone help?

' This SolidWorks VBA macro sets the "dn" custom property based on the
' first three characters after "DET-" in the file name of the open part.
Sub main()
    Dim swApp As SldWorks.SldWorks
    Dim swModel As SldWorks.ModelDoc2
    Dim swCustPropMgr As CustomPropertyManager
    Dim fileName As String
    Dim dnValue As String
    Dim detPos As Long

    Set swApp = Application.SldWorks
    Set swModel = swApp.ActiveDoc
    If swModel Is Nothing Then
        MsgBox "No active document!"
        Exit Sub
    End If

    fileName = swModel.GetPathName
    fileName = Mid(fileName, InStrRev(fileName, "\\") + 1)
    detPos = InStr(fileName, "DET-")
    If detPos = 0 Then
        MsgBox "Pattern 'DET-' not found in file name."
        Exit Sub
    End If
    dnValue = Mid(fileName, detPos + 4, 3)

    Set swCustPropMgr = swModel.Extension.CustomPropertyManager("")
    Call swCustPropMgr.Add3("dn", swCustomPropertyText, dnValue, swCustomPropertyReplaceValue)

    MsgBox "Property 'dn' set to '" & dnValue & "'"
End Sub