I was placing down a red underground belt (the first of the pair) and got this error. Happens every time I try to place a red underground belt in 0.15.13. Didn't happen in 0.15.12. I disabled all other mods to make sure it was just orphan finder, and still get the issue even with no other mods.

Error while running event Orphan Finder::on_built_entity (ID 6)
Orphan Finder/control.lua:39: bad argument #1 to 'pairs' (table expected, got nil)

I hit the same bug, in 0.15.13, same time as original poster, when placing underground red belts, modded in my case. It never happened in .15 - .15.12 .

Seems to be related to this bit of .15.13 ChangeLog

Fixed LuaEntity::neighbours return format to match the docs. (48176)

Postby Rseding91 » Mon May 22, 2017 2:12 am
Making it return what the docs say so "nil" or the entity for underground belts.

Got same error:
[img]http://i.imgur.com/BhJ3EDt.png[/img]

Got the same error with all underground belts, the pipes worked fine

Thanks for the Mod

Seems to be related to this bit of .15.13 ChangeLog

Fixed LuaEntity::neighbours return format to match the docs. (48176)

Postby Rseding91 » Mon May 22, 2017 2:12 am
Making it return what the docs say so "nil" or the entity for underground belts.

It's slightly more complex than that, now underground belts return a single entity while undergound pipes return an array. Will fix when I get home from work tonight.

control.lua:68:
change to

for _,neighbour in pairs(pipe.neighbours[1]) do

fixed :)