That's seriously a WTF then. The only functions that raise that exception are string and array get/set, and it's compiled with unsafe, which turns those off. Also, I can't reproduce at all with unsafe turned on for ocamlopt, even after installing 3.08.1 on my system.
I've submitted a version that explicitly uses String.unsafe_get, which in my mind proves that the -unsafe flag isn't being passed/picked up correctly for some reason.