Getting Null error-- getReviewUserIdentity.ProviderUserKey

If I log in to default membership provider, I don't get null on getReviewUserIdentity.ProviderUserKey.
However, if I log in non-default membership provider, I get null error although I'm authenticated.

Do I have to specify membership provider in order to use getReviewUserIdentity.ProviderUserKey??
Any idea what's going on?



Dim reviewUserID As String
 If User.Identity.IsAuthenticated Then
            Dim getReviewUserIdentity As MembershipUser = Membership.GetUser()
            reviewUserID = getReviewUserIdentity.ProviderUserKey.ToString()
Else
            reviewUserID = 0
End If
...