asp.net DROP DOWN LIST

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ASP.NET

Public Class CommonDropDownList
        Inherits System.Web.UI.Page
        Sub New()
        End Sub
        Sub FillDropDownList(ByVal myDropDown As DropDownList, ByVal SqlString As String, ByVal txtName As String, ByVal valueName As String, ByVal extraRow As String, ByVal extraRowVal As String)
                      ConnectDB()
            Conn.Open()
            Dim myCommand As New OleDbCommand(SqlString, Conn)
                       myDropDown.DataSource = myCommand.ExecuteReader()
            myDropDown.DataTextField = txtName
            myDropDown.DataValueField = valueName

            myDropDown.DataBind()
            Conn.Close()
            If Not extraRow = "" Then
                myDropDown.Items.Insert(0, New ListItem(extraRow, extraRowVal))
                myDropDown.SelectedIndex = 0
            End If
        End Sub


This is a common code and I am using the following code in the page load 

If Not IsPostBack Then


            sqlstr = "select lpad(level,2,'0')mth from dual connect by level<13"
            BindDDL.FillDropDownList(ddlMth, sqlstr, "mth", "mth", "Select", "Select")
            sqlstr = "select (to_char(sysdate,'yyyy')-level+1)yr from dual connect by level<6"
            BindDDL.FillDropDownList(ddlYear, sqlstr, "yr", "yr", "Select", "Select")


        End If

I am getting the Null Exception on running the code though the individual queries are running perfectly fine...How can I correct it please help