Ошибка IndexError

Заранее извиняюсь за криво оформленный вопрос. У меня лапки, и вообще я здесь новенький. Можете мне подсказать почему выдает эту ошибку?

IndexError: index -9999 is out of bounds for axis 0 with size 16

import numpy as np 
import matplotlib as mpl 
import matplotlib.pyplot as plt 
from scipy import linalg, optimize
from scipy.spatial.distance import pdist, squareform 
from scipy.sparse import csr_matrix
from scipy.sparse.csgraph import dijkstra

word_list = open('dict1.txt').readlines()
word_list = map(str.strip, word_list)
word_list = [word for word in word_list if len(word) == 3]
word_list = [word for word in word_list if word[0].islower()]
word_list = [word for word in word_list if word.isalpha()]
word_list = list (map (str.lower, word_list))
# print(len(word_list))

word_list = np.asarray(word_list)
word_list.sort() 
# print(word_list.dtype)

word_bytes = np.ndarray((word_list.size, word_list.itemsize),
                        dtype='uint8',
                        buffer=word_list.data)
word_bytes = word_bytes[:, ::word_list.itemsize//3]
# print(word_bytes.shape)

hamming_dist = pdist (word_bytes, metric = 'hamming')
graph = csr_matrix (squareform (hamming_dist < 1.5 / 3))
i1 = word_list.searchsorted ('ape')
i2 = word_list.searchsorted ('man')
distances, predecessors = dijkstra(graph, indices=i1,
                                   return_predecessors=True)
# print(distances[i2])

path = []
i = i2
while i != i1:
    path.append(word_list[i])
    i = predecessors[i]
path.append(word_list[i1])
print(path[::-1]) 

файл dictt1.txt

imp
amp
asp
ass
ads
ids
ins
inn
ion
won
woo
who
oho
ohm
jopo
good
super
man
capuchino
aqua
the
world
ags
vnm
try