Getting the SID from the current user

Hi,

I've a general question retrieving the SID of the current user.

When fetch the SID with this following code I getting an empty value:

SPContext.Current.Web.CurrentUser.Sid

Alternatively I get successfully the SID with the second code snippet:

SPContext.Current.Web.CurrentUser.UserId.NameId

Why is the direct SID value empty?

Any help appreciated.