you are correct; you are testing for five specific values in your CASE, and there's no explicit ELSE in it to handle other values, and the built in logic structure of the case returns NULL for items that don't match the tests in the CASE when no explicit ELSE is provided.
help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!