Class events question

[learning]

I set up a class with an event as shown here :

Imports System.IO.Ports
Public Class CommPort
    Dim _msg As String
    Dim WithEvents SerialPort As New SerialPort
    Public Sub New(ByVal portID As String)
        If SerialPort.IsOpen Then
            SerialPort.Close()
        End If

        Try
            With SerialPort
                .PortName = portID
                .BaudRate = 9600
                .Parity = IO.Ports.Parity.None
                .DataBits = 8
                .StopBits = IO.Ports.StopBits.One
            End With
            SerialPort.Open()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

    Public Event DataReceived(ByVal message As String)

    Private Sub ReadData(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort.DataReceived
        Dim _bytes As Integer = SerialPort.BytesToRead
        _msg = ""
        For i As Integer = 0 To _bytes - 1
            _msg = String.Concat(_msg, Chr(SerialPort.ReadChar))
        Next
        RaiseEvent DataReceived(_msg)
    End Sub
End Class

And from the main program am using this code :

Public Class Main
    Dim WithEvents commTest As New CommPort("COM1")

Private Sub ProcessCommData(ByVal message As String) Handles commTest.DataReceived
        txtSONumber.Text = message
    End Sub
End Class

When the event triggers I get an error in the program where trying to update the forms text that says "Cross-thread operation not valid: Control 'txtSONumber' accessed from a thread other than the thread it was created on."

Am I using the custom event incorrectly?

Thanks in advance for your help.