Created Unassigned: renci.sshnet - Problem with execute su - user [2623]

Hi all, i write this code for connect with a server in SSH with vb2010:

Public Class Form1

'Create the objects needed to make the connection'
Dim connInfo As New Renci.SshNet.PasswordConnectionInfo("10.177.192.19", "admin", "pwd")
Dim sshClient As New Renci.SshNet.SshClient(connInfo)
'Need to hold the command'
Dim cmd As Renci.SshNet.SshCommand
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'Using sshClient
sshClient.Connect() 'connect to the server'
cmd = sshClient.RunCommand("hostname")
lblserver.Text = cmd.Result
Me.Refresh()

cmd = sshClient.RunCommand("su - guest")
If cmd.Error <> "" Then
TextOutput.Text = cmd.Error
sw.WriteLine(Replace(cmd.Error, Chr(10), vbCrLf))
Else
TextOutput.Text = Replace(cmd.Result, Chr(10), vbCrLf)
sw.WriteLine(Replace(cmd.Result, Chr(10), vbCrLf))
End If
sshClient.Disconnect()

end sub

The problem occurs when you run the line "cmd = sshClient.RunCommand("su - guest")", the program stops responding and does not go forward.
Why?
Help me please...
Thank's