Error consuming WebService with Vb.Net

I am consuming a web service, by giving the url in the browser if I see the xml with all its methods but when I consume it, it throws this error:

  

The server was unable to process the request due to an internal error.   For more information about the error, either turn on   IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute   or from the configuration behavior) on the server in   order to send the exception information back to the client, or turn on   tracing as per the Microsoft .NET Framework SDK documentation and   inspect the server trace logs.

The client code I have is:

Private Sub btnPruebaFirma_Click(sender As Object, e As EventArgs) Handles btnPruebaFirma.Click
    Try
        Dim ArchivoCerByte As Byte()
        Dim ArchivoKeyByte As Byte()
        ArchivoCerByte = File.ReadAllBytes("ArchivoCer")
        ArchivoKeyByte = File.ReadAllBytes("ArchivoKey")
        Dim WS As New ServicioDeFEAClient()
        Dim Firmante As ServicioDeFEA.FirmanteO = New ServicioDeFEA.FirmanteO()
        Firmante.RFC = "RFCRFCRFCRFC"
        Firmante.Area = "AREAX"
        Firmante.CargoFirmante = ""
        Firmante.Certificado = ArchivoCerByte
        Firmante.Key = ArchivoKeyByte
        Firmante.Password = "12345678"
        Firmante.EntidadCertificadora = 1
        Dim Cadena(1) As String
        Cadena(0) = "a"
        Cadena(1) = "b"
        Dim Firmas As Array = WS.FirmaCadena(Cadena, Firmante)
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    End Try
End Sub