Custom ExtendedMembershipProvider with Guid instead of int for UserId

Hi have an existing MembershipProvider which I am extending to inherit from ExtendedMembershipProvider. I use Guid for UserId instead of int.
Unfortunately, functions such as GetUserIdFromOAuth and GetUserNameFromId all assume that the unique identifier is an int. This seems like quite an oversight (given that MembershipProvider used object for ProviderId).
Is there any workaround to this?
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