When I run this on my admittedly very complex object model, I get an uncaught exception:
UNCAUGHT EXCEPTION (NSInvalidArgumentException): *** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]
I've attached the entire crash report.

I love this tool! I successfully ran it on this project once a while ago, and was able to give people who have never touched this code an idea of how complex it is.

ObjectGraph_crashlog.txt