Recovering value null

I have the following problem, I do not know why I can not receive the value of my quotation code, the same comes null, and in the database I have a quotation table that has several quotation codes. the value is receiving value type 0.

Follow my codes:

private int codcotation;

        [Id (Column = "COD_COTATION", Name = "Code")]
        [Generator (1, Class = "sequence")]
        [Param (2, Name = "sequence", Content = "SDP_COD_COTATION_SEQ")]

        public virtual int Code
        {
            get {return codcotation; }
            set {codcotation = value; }
        }

protected void btn AddItemExecution_Click (object sender, ImageClickEventArgs e)
        {

           
             if (codcotation.Value == String.Empty)
                    codcotation.Value = String.Empty;

        }

public static QueryPorId (int idCotation)
        {
            ISession session = NhibernateHelper.
            Quotation quotation = null;
            try
            {
                quotation = (Quotation) session.Load (typeof (Quotation), idCotation);
                return quotation;
            }
            catch (Exception e)
            {
                throw new Exception (e.ToString ());
            }
        }

{"ORA-01400: Can not insert NULL into (". \" SDPE_ITEM_EXECUTION \ ". \" COD_COTATAION \ ")}}

Help me friends, I do not know what I do because I can not get the values ​​that come from my database.