HttpContext.Current.User.Identity.Name Blank ?

I have implemented the forms authentication method using  MSDN web site http://msdn2.microsoft.com/en-us/library/ms160724.aspx

and now i want to modify that code

so when i used

Dim UserName as string =HttpContext.Current.User.Identity.Name

from the vb.net code behind

it gives me ss as blank value ?

but the same code i used from the normal web site its gives me the

"SER1Administrator" value for that string(current user)

so my problem is why is that code not working for the report server sample project ,which microsoft provided ?

do i have to change any configuration settings ?

please help me .......i am in trouble ?

(what i really want is i need to get the username from the active page directory? wheather this username exist from the active page directory.)

 

regards

sujithf