Outlook - attachment

Fra min database kan man sende mail via Outlook. Men der er et problem, jeg vil helst gerne checke om der ER et attachment eller ej, men nedenstående kode går i fejl og viser "Type mismatch". Når jeg kører i Debug mode, viser markøren over Attachment teksten "missin".
Function SendMail(Recipients As String, Subject As String, MessageText As String, Optional Attachment As Variant, Optional CC As String, Optional BCC As String)
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    If Attachment <> "" Then
      OutMail.Attachments.Add (Attachment)
    End If
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End Function