full text searching on Membership tables

Hi, Can anyone explain or point me in the right direction as to why I cannot set up a full text index on any of the aspnet_ Membership tables.  Enterprise manager denies attempting to set an index up and scripting the full text index fails with a non deterministic key error? I must have this scripted.

IF NOT EXISTS(
SELECT *
FROM sys.fulltext_indexes
JOIN sys.tables
ON sys.tables.object_id = sys.fulltext_indexes.object_id
WHERE sys.tables.name = 'aspnet_Users'
)
CREATE FULLTEXT INDEX ON [dbo].[aspnet_Users] ([Lo ...