Guido, that's no good.  The outer loop is traversing a doubly-linked circular list, and it should be flatly impossible for gc to ever be NULL - the list structure is insanely damaged if any gc_next or gc_prev field reachable from it is NULL (gc always comes from a gc_next or gc_prev field)

I'd wonder whether the `collectable` argument passed in was NULL to begin with, but Antoine's traceback shows that it's not.

So, like most crashes in gc, we're just seeing the end symptom of something that went wrong long before.  Guarding against gc != NULL is just hiding this particular symptom.