I need an expert in here! International Trade Routes mod

Part of my mod, which includes a growing/spreading strategic resource (which, rather than being improved, is scrubbed for a production boost for the nearest city), involves distributing production between cities via domestic trade routes. This doesn't conjure up production but subtracts it from the donor city. As a result, trade routes in the network are dependent on others, and so, should have their effects calculated after the previous ones in the chain. I have attached a doc which outlines my algorithm for determining trade route dependency, as well as my code.

Thing is, my code doesn't work. There is an error at line 470, saying that I am attempting to field index ?, a nil value. I cannot for the life of me figure out why. My code is long, so any attempts to help would be greatly appreciated!

Anyone?