Check for null values in CAML query

I have a custom list named "Custom" which has the following columns.

1.) Title
2.) Test

I need to use CAML query and get only the items which has null values in "Test" column.

Code snippet:

 using System;using System.Collections.Generic;using System.Linq;using System.Text;using Microsoft.SharePoint;using System.Data;using System.Globalization; namespace ISNULL{    class Program    {        static void Main(string[] args)        {            using (SPSite site = new SPSite("http://serverName/sites/Vijai"))            {                using (SPWeb web = site.OpenWeb())                {                    SPList list = web.Lists.TryGetList("Custom");                    SPQuery query = new SPQuery();                    query.Query = "<Where><IsNull><FieldRef Name='Test' /></IsNull></Where>";                    SPListItemCollection itemColl = list.GetItems(query);                    foreach (SPListItem item in itemColl)                    {                        Console.WriteLine(item["Title"]);                    }                    Console.ReadLine();                                  }            }        }    }}