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();                                  }             }         }     } }