the above error i am getting wen i am executing thi program..... plz help me

public with sharing class ChatterUnitTests {

/*
* Test the ChatterActivity trigger which inserts chatter feed posts whenever a task is inserted on the parent object
*/
public static testMethod void testChatterActivity() {
//create the test account

Account a = new Account(name='Test Account');
insert a;

//create a task on that account
Task t = new Task(whatId=a.id);
t.Subject = 'This is a test activity for chatter';
t.ActivityDate = System.today();
t.Status = 'In Progress';
t.Description = 'Hello, this will be chattered';

insert t;

//Make sure Account has the feed enabled. If it does, make sure the chatter feed post is there
Schema.DescribeSObjectResult r = Account.SObjectType.getDescribe();
if (r.isFeedEnabled()) {
List<AccountFeed> posts = [SELECT Id, Type FROM AccountFeed WHERE ParentId = :a.id];
System.assertEquals(1, posts.size());
}
}
}