[Solved] ASP.Net Error: NvarChar is not member of NpgsqlTypes.NpgsqlDbTypes

NvarChar is not member of NpgsqlTypes.NpgsqlDbTypes ..But i import Namespace

Imports Npgsql
Imports System.Collections.Generic
Imports System.Configuration
Imports System.Web.Services
Imports NpgsqlTypes

Public Class GraphData
    Public Property label As String
    Public Property value As Integer
End Class

<WebMethod()>
Public Shared Function GetGraphData(ByVal customerId As String) As List(Of GraphData)
    Dim graphDataList As List(Of GraphData) = New List(Of GraphData)()
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim con As NpgsqlConnection = New NpgsqlConnection(constr)
    Dim cmd As NpgsqlCommand = New NpgsqlCommand("SELECT TOP 10 CAST(OrderDate AS DATE) OrderDate, SUM(Freight) Freight FROM Orders WHERE CustomerID = @Id OR @Id IS NULL GROUP BY Orderdate", constr)
    cmd.Parameters.Add(New NpgsqlParameter("Id", NpgsqlDbType.NVarChar))
    cmd.Parameters("Id").Value = If(Not String.IsNullOrEmpty(customerId), customerId, CObj(DBNull.Value))
    cmd.Connection = con
    con.Open()
    Dim sdr As NpgsqlDataReader = cmd.ExecuteReader()

    While sdr.Read()
        graphDataList.Add(New GraphData() With {
            .label = Convert.ToString(sdr("OrderDate")),
            .value = (Convert.ToInt32(sdr("Freight")))
        })
    End While

    con.Close()
    Return graphDataList
End Function

<asp:DropDownList ID="ddlCustomerID" runat="server">
    <asp:ListItem Text="Maria" Value="ALFKI"></asp:ListItem>
    <asp:ListItem Text="Ana Trujillo" Value="ANATR"></asp:ListItem>
    <asp:ListItem Text="Antonio Moreno" Value="ANTON"></asp:ListItem>
    <asp:ListItem Text="Thomas Hardy" Value="AROUT"></asp:ListItem>
    <asp:ListItem Text="Christina Berglund" Value="BERGS"></asp:ListItem>
</asp:DropDownList>
<div id="line-chart">
</div>
<link type="text/css" rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/morris.js/0.5.1/morris.css" />
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/raphael/2.1.0/raphael-min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/morris.js/0.5.1/morris.min.js"></script>
<script type="text/javascript">
    $(document).ready(function () {
        BindMorrisChart('');
        $('[id*=ddlCustomerID]').on('change', function () {
            var id = $(this).val();
            BindMorrisChart(id);
        });
    });
    function BindMorrisChart(id) {
        $('#line-chart').empty();
        $.ajax({
            type: 'POST',
            url: "Default.aspx/GetGraphData",
            data: '{ "customerId": "' + id + '" }',
            dataType: 'json',
            contentType: "application/json; charset=utf-8",
            success: function (result) {
                new Morris.Donut({
                    element: 'line-chart',
                    data: result.d,
                    xkey: 'label',
                    ykeys: ['value'],
                    labels: ['value']
                });
            },
            error: function (error) {
                alert(error.responseText);
            }
        });
    }
</script>