FIRE run time error

So I need help troubleshooting an NZEC error. My code works for all cases I have tried. I’ve even written code that generators random trees of up to 10000 bushes w/ rdm initial starting points and bushes to save. Again, my code works every time. The only things I can think of are: 1) There is some obscure corner case that I have not identified and would never occur in a random tree, 2) There is a problem with me writing in python 3.4 and the interpreter used here is 3.1 3) The data provided is not what my program expects, but, again, I have run a ton of test data according the problem description w/ no problems. Can anyone shed some light on this for me? For reference, my code is here: