Is this error message appearing as you launch? The main entry points I'm finding that make that call check the input in a way that should prevent this error, but there must be a codepath I'm not seeing. What version of Character Animator are you using and on what operating system? Presuming I'm guessing correctly that you are seeing it at launch, how are you launching it? (double clicking a puppet or project in Finder/Explorer, clicking an icon/shortcut for the app itself, etc) Is this the first time you're running it? (startup is slightly different the first time due to not necessarily having preferences or an indication of the last open project)