How to validte a user name for delicious?

I am writing a rss monitoring service with the universal feed parser,
but I found that for invalid user_name (try:
http://del.icio.us/rss/abcdfdfd_abcefdkfj1233333), delicous still
returns as an empty rss (?).

So I am wondering whether there is a decent way of validating user name?

side note: I know I can do something to check
http://del.icio.us/abcdfdfd_abcefdkfj1233333, but it seems pretty messy?)