UDP sending problem

[Mr. Blank]

Hi

I'm working on a messenger that uses UDP and i can send a message once alright, but when i try and send a second message i get an error "An invalid argument was supplied" for this line

SendUdp.JoinMulticastGroup(GroupIP, 12)

I have no idea why this is happening (not that i know that much about udp anyway), so if anyone could help out that would be well appreciated. Thanks.

This is called when enter key is pressed

Private Sub UDPSend()

        GroupIP = IPAddress.Parse("224.0.0.0")
        GroupEP = New IPEndPoint(GroupIP, 2056)
        SendUdp.JoinMulticastGroup(GroupIP, 12) 'error happening here 2nd time through

        Dim bteSendDate() As Byte

        Try
            bteSendDate = Encoding.Unicode.GetBytes(rtb_MessageWrite.Text)
            SendUdp.Send(bteSendDate, bteSendDate.Length, GroupEP)

        Catch ex As Exception
            rtb_MessageView.Text = rtb_MessageView.Text & (ex.Message)
        End Try

    End Sub

This is on a new thread

Private Sub BeginListen()
        Dim bteReceiveData() As Byte
        Dim strReceiveData As String

        GroupIP = IPAddress.Parse("224.0.0.0")

        GroupEP = New IPEndPoint(GroupIP, 2056)
        ListenUdp = New UdpClient(2056)
        ListenUdp.JoinMulticastGroup(GroupIP)

        Do
            bteReceiveData = ListenUdp.Receive(GroupEP)
            strReceiveData = Encoding.Unicode.GetString(bteReceiveData)

            If rtb_MessageView.InvokeRequired Then
                Dim d As New SetTextCallback(AddressOf SetText)
                Me.Invoke(d, New Object() {strReceiveData})
            Else
                rtb_MessageView.Text = rtb_MessageView.Text & strReceiveData
            End If
        Loop

    End Sub

    Private Delegate Sub SetTextCallback(ByVal a As String)

    Private Sub SetText(ByVal [text] As String)
        rtb_MessageView.Text = rtb_MessageView.Text & [text]
    End Sub