That is so, yes. Since arc can change at any time, to use it you have to be open to changing your app anytime you pull in updates. It's one of the reasons arc is small; it has no bloat from historical baggage. I've given up and even keep my own code directly in the arc directory.
I'm curious what your current setup is. Are you running off of anarki? Or copying parts of it over to your own repo?
I just looked in Arc 3.1, in all of Anarki's branches, a few commits back in Anarki's master ac.scm, and I didn't find anything at line 972 that would cause an error like that.