VB.NET มีปัญหาในใช้ CType กับ Chart ครับ รบกวนผู้รู้แนะนำด้วยครับ

For i As Integer = 1 To 5
        Sql = "select type_dt,usetime " & _
                 "from Logdowntime " & _
                 "where id_dep = '" & i & "' " & _
                 "group by type_dt "
        Dim da As New SqlDataAdapter(Sql, objConn)
        Dim ds As New DataSet
        da.Fill(ds, "tb")

        '*** BindData to Charts ***'
        Dim cht As New Chart
        cht = CType(Me.Controls("Chart" & i), Chart)

        With cht 
            .DataSource = ds.Tables("tb")   '<<< มันขึ้น Error ที่บรรทัดนี้ะครับ Object reference not set to an instance of an object.
            .Series("Series1").ChartType = SeriesChartType.Column
            .Series("Series1")("DrawingStyle") = "Emboss"
            .Series("Series1").XValueMember = "type_dt"
            .Series("Series1").YValueMembers = "usetime"
            .Series("Series1").IsValueShownAsLabel = True
            .Series("Series1").IsVisibleInLegend = False
            .Size = New System.Drawing.Size(325, 300)
            .DataBind()
        End With
Next