Type erro webclient

Question

Boa noite amigos e amigas, bom estou com esse erro quem sabe porque ? Obrigado.

Codigo 

    Dim WithEvents x As New WebClient
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim open As New OpenFileDialog
        With open
            If .ShowDialog = Windows.Forms.DialogResult.OK Then
                Dim url As String = "site/post.php?web=1"
                Dim w As New NameValueCollection
                w.Add("type", .FileName)
                w.Add("name", IO.Path.GetFileName(.FileName))
                x.QueryString.Add(w)
                x.UploadFileAsync(New Uri(url), "post", .FileName)
            End If
        End With
    End Sub

Erro !

{ "type": "error" }