NTAccount.Translate to SecurityIdentifier for <domain>\Administrat

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Hi!
Can anyone please explain me why following code gives an SecurityException
"System.Security.SecurityException: The name provided is not a properly
formed account name."?

-- code start --

System.Security.Principal.NTAccount ntAccount = new
System.Security.Principal.NTAccount(@"<domain>\Adm inistrators");
System.Security.Principal.SecurityIdentifier sid =
(System.Security.Principal.SecurityIdentifier)ntAc count.Translate(typeof(System.Security.Principal.S ecurityIdentifier));

-- code end --

where "<domain>" in "<domain>\Administrators" is your Active Directory Domain.

Note:
<domain>\Administrators is a built-In group. It is visible trough 'Find'
dialog in Active Directory Users and Computers Snap-In.

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up

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please help!