Can you see the problem with my ctype(string,dropdownlist), Object reference not set to an instance of an object

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I know that it is something small, I just don't want to write the same code over and over again... 
Basically I want to loop through and assign the dropdownlists (ddDocType0-ddDocType9)

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
        If Not Page.IsPostBack Then

     Dim i As Integer
            For i = 0 To 9
                Dim dd As DropDownList
                dd = CType(Page.FindControl("ddDocType" & i.ToString()), DropDownList) --this is where the problem is...
                dd.Items.Add("Select ...