Yahoo Mail nicknames don't work?

I have had my Yahoo email account for around 2 years now, and just recently (perhaps a week ago), my nicknames have stopped working. Whenver I try to send an email typing the nicknames into the "To" field, I get a message saying a syntax error occurred. The only way I can send an email is by typing in the recipient's whole email address. What can I do to be able to use nicknames again?