Outlook VBA Errors and disables rule

This VB script looks at the begining of the subject and removes the mess that our IT dept adds to our email to notify us that the email came from an external source and should be handled with care.  Since I'm smart enough to do that without being reminded I personally want to remove that because an email conversation will end up with an additional string pre-pended to the subject line.    "RE: <EXT> Subject RE: <EXT> SubjectRE: <EXT> Subject RE: <EXT> Subject RE: <EXT> Subject RE: <EXT> Subject <EXT> Subject"  

Occasionally it throws an error that I haven't been able to trap and handle which causes the rule to be deactivated.  Any suggestions?

Sub Remove_EXT(myItem As MailItem)

    On Error GoTo Error_Handler
    
    Dim mySubject As String
    mySubject = myItem.Subject

If (Left(mySubject, 6)) = "<EXT> " Then
    mySubject = Right(mySubject, Len(mySubject) - 6)
End If

If (Left(mySubject, 5)) = "<EXT>" Then
    mySubject = Right(mySubject, Len(mySubject) - 5)
End If

If mySubject = "" Then
    Exit Sub
Else
       myItem.Subject = mySubject
       myItem.Save
End If

Exit Sub

Error_Handler:

    MsgBox "An error occurred" & vbCrLf & vbCrLf & "Error Number: " & Err.Number & vbCrLf & _
    "Error Source: RemoveExternalPrefix " & "Error Description: " & Err.Description, vbCritical, "An Error has Occured!"
    
End Sub

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