SerialPort.open() - error message: COM does not exist

Code:

---

#Region "Manager Variables"
    'property variables
    Private _baudRate As String = String.Empty
    Private _parity As String = String.Empty
    Private _stopBits As String = String.Empty
    Private _dataBits As String = String.Empty
    Private _portName As String = String.Empty
    Private _transType As TransmissionType
    Private _msg As String
    Private _type As MessageType
    'global manager variables
    Private comPort As New SerialPort()
    Private write As Boolean = True
#End Region

---

Code:

---

    Public Function OpenPort() As Boolean
        Try
            'first check if the port is already open
            'if its open then close it
            If comPort.IsOpen = True Then
                comPort.Close()
            End If

            'set the properties of our SerialPort Object
            comPort.BaudRate = Integer.Parse(_baudRate)
            'BaudRate
            comPort.DataBits = Integer.Parse(_dataBits)
            'DataBits
            comPort.StopBits = DirectCast([Enum].Parse(GetType(StopBits), _stopBits), StopBits)
            'StopBits
            comPort.Parity = DirectCast([Enum].Parse(GetType(Parity), _parity), Parity)
            'Parity
            comPort.PortName = _portName
            'PortName

            'now open the port
            comPort.Open()
            'display message
            _type = MessageType.Normal
            _msg = "Port opened at " + DateTime.Now + "" + Environment.NewLine + ""
            MsgBox(_type, _msg)
            'return true
            Return True
        Catch ex As Exception
            MsgBox(MessageType.[Error], ex.Message)
            Return False
        End Try
    End Function

---

I have a catch ex As Exception at comport.Open() with error message saying "COM18 does not exist". I checked with serialport.getportnames() that it is listed. It is also listed on device manager.

The port settings I checked were correct:
comPort.BaudRate = 9600
comPort.DataBits = 8
comPort.StopBits = 1
comPort.Parity = None {0}
comPort.PortName = "COM18"

Am I missing something?

Thanks