TypeError: undefined has no properties in compare_entities?

[1j01]

I got some warnings when running create_arrow_test_scene() in the console, in a non-empty world, and then when I went into edit mode I got an error TypeError: undefined has no properties

It's supposedly happening here:

            for (k = 0, len = relative_sorts.length; k < len; k++) {

in compare_entities
but relative_sorts looks fine, it's an array of length 7. And that's the only property access on that line.
Maybe a bug with Firefox dev tools?