April CTP superkey oddity

I've set up a table with a superkey composed of a uniqueidentifier column and a varchar column. The uniqueidentifier column is not set to be a row GUID. When I try to add a new row with a duplicate uniqueidentifier value but different varchar value, I am told it violates the IX uniqueness constraint. I can't figure out what's wrong with this since it looks valid to me. Thanks in advance for suggestions.

JoshJust an FYI- I dropped the table and recreated it as I did before and everything went as expected!

Josh