'NoneType' object has no attribute 'key' error in decrease_key

[gracejenkins]

I'm using fibonacci heaps for a priority queue in a min cut algorithm. I've been able to successfully implement it with a couple of test graphs, but suddenly for one of my tests I'm getting this issue in decrease_key:

Traceback (most recent call last):
File "TSPSolver.py", line 248, in
main('test2.txt')
File "TSPSolver.py", line 244, in main
tsp = remove_subtours(tsp,G)
File "TSPSolver.py", line 200, in remove_subtours
constraint_edges, min_cut_nodes = find_subtour(G, model.X)
File "TSPSolver.py", line 182, in find_subtour
min_cut_nodes, min_cut_value = min_cut(G_x)
File "TSPSolver.py", line 156, in min_cut
cut_of_phase, cut_nodes, G = min_cut_phase(G)
File "TSPSolver.py", line 144, in min_cut_phase
to_search = update_fibheap(G,to_search, heap_nodes, a.value)
File "TSPSolver.py", line 124, in update_fibheap
fibheap.decrease_key(node,node.key - change_in_weight)
File "/Users/ArluahsMAC/Desktop/CAAM471PTSD/fib_heap.py", line 76, in decrease_key
if x.key < self.min_node.key:
AttributeError: 'NoneType' object has no attribute 'key'

Any idea why this might be happening?
Thanks!