GetUser() returns null even for authenticated user

Even when I have logged in, and, as the code below shows, am authenticated and online, GetUser()  with no overloads returns NULL.     HELP PLEASE..!
?User.Identity
AuthenticationType: "Forms"
IsAuthenticated: true

?Membership.GetUser("mgp22")
    CreationDate: {7/29/2009 4:38:29 PM}
    IsApproved: true
    IsLockedOut: false
    IsOnline: true
    ProviderName: "AspNetSqlMembershipProvider"
    UserName: "mgp22"


UPDATE:  For some reason, after I deleted and recreated the accounts in the Web Site Admin tool, the problem has gone away...  Don't know why!  but happy .

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