MessageBox causes "COM object that has been separated from its underlying RCW"

I have a problem where the MessageBox function causes an Outlook Item object to be released (COM object that has been separated from its underlying RCW). The following code works fine if I comment out the MessageBox function, but produces an error when it is included. Can anyone explain what I am doing wrong?

 Public Sub TestMsgBox(p_item As Object)
        Dim z_type As String = Nothing
        Dim z_result As Integer = 0
        If Not IsNothing(p_item) Then
            Try
                z_result = MessageBox.Show("An item has been passed")
                If TypeOf p_item Is Outlook.AppointmentItem Then
                    z_type = "Appointment Item"
                End If
            Catch ex As Exception
                System.Diagnostics.Debug.Print("ERROR")
            End Try
        End If
    End Sub

MikePL