Issue with serial port in VS 2005

I am making an app that uses a serial port to listen for incoming data, but for some reason I cannot get it to work. It freezes when I click the button to start the app. Any ideas??

Thanks,

Brad

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Imports System.IO


Public Class Form1


    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load


    End Sub

    Function ReceiveSerialData() As String
        ' Receive strings from a serial port.
        Dim returnStr As String = ""

        Using com4 As IO.Ports.SerialPort = _
                My.Computer.Ports.OpenSerialPort("COM4")
            Do
                Dim Incoming As String = com4.ReadLine()
                If Incoming Is Nothing Then
                    Exit Do
                Else
                    returnStr &= Incoming & vbCrLf
                End If
            Loop
        End Using

        Return returnStr
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ReceiveSerialData()
    End Sub
End Class