Problem with authenticating user using Active Directory

Hello everyone,
I have a question about authenticating a user in Classic asp. I have this application running on a network. It works fine for every user except one. His username is coming back null. Because it works fine for everyone else, myself included, I'm inclined to think it's not the code but either the users account and how it's being authenticated or its his browser settings. If anyone can point me in the right direction, I'd appreciate it. The code that is causing the problem is:

This variable returns a username for everyone who visits the page with the exception of one particular user. His account exists in our Active Directory but I'm not sure where else to look

Thanks for any help

jason