Postgresql插入C#后出现强制转换异常

使用此代码，我在与参数categoryName相对应的表中搜索id。如果不存在，我在表中插入值，然后返回键。当执行long idCategory = (long) id;时，有时它会工作，有时会引发无效的强制转换异常。为什么？？？

    public static long GetCategoryID(String categoryName)
    {
        using (NpgsqlConnection conn = new   NpgsqlConnection(ConfigurationManager.ConnectionStrings["myConn"].ToString()))
        {
            conn.Open();
            NpgsqlTransaction transaction = conn.BeginTransaction();

            String query = "SELECT idcategory FROM category WHERE categoryname = '" + categoryName + "';";
            NpgsqlCommand cmd = new NpgsqlCommand(query, conn, transaction);

            object id = cmd.ExecuteScalar();
            if (id == null)
            {
                query = "INSERT INTO Category (categoryname) VALUES ('" + categoryName +"');";
                cmd = new NpgsqlCommand(query, conn, transaction);
                cmd.ExecuteNonQuery();
                query = "SELECT LASTVAL();";
                cmd = new NpgsqlCommand(query, conn, transaction);
                id = cmd.ExecuteScalar();
            }
            long idCategory = (long)id;
            transaction.Commit();
            conn.Close();
            return idCategory;
        }
    }