I'm aware of this issue and need to code around it. This usually means that you haven't set up the burner drive in the preferences. I will try and get it fixed so that it will warn you and make you set it before trying to burn or blank a disk.
OK, Yogi, I looked into it and yes there was a bug. It will be fixed in the next Grafburn snapshot, and I'm uploading a new Grafburn to the forum in the additional software section. I inadvertantly introduced the bug when I was fixing a different one. Thanks again for reporting it.