ValueError: max() arg is an empty sequence

Here is the error I am getting:
(env3.6) khashab2@dickens:~/ideaProjects/e2e-coref$ python demo.py final

[nltk_data] Downloading package punkt to /home/khashab2/nltk_data...
[nltk_data]   Package punkt is already up-to-date!
Setting CUDA_VISIBLE_DEVICES to: 
Running experiment: final
max_top_antecedents = 50
max_training_sentences = 50
top_span_ratio = 0.4
filter_widths = [
  3
  4
  5
]
filter_size = 50
char_embedding_size = 8
char_vocab_path = "char_vocab.english.txt"
context_embeddings {
  path = "glove.840B.300d.txt"
  size = 300
}
head_embeddings {
  path = "glove_50_300_2.txt"
  size = 300
}
contextualization_size = 200
contextualization_layers = 3
ffnn_size = 150
ffnn_depth = 2
feature_size = 20
max_span_width = 30
use_metadata = true
use_features = true
model_heads = true
coref_depth = 2
lm_layers = 3
lm_size = 1024
coarse_to_fine = true
max_gradient_norm = 5.0
lstm_dropout_rate = 0.4
lexical_dropout_rate = 0.5
dropout_rate = 0.2
optimizer = "adam"
learning_rate = 0.001
decay_rate = 0.999
decay_frequency = 100
train_path = "train.english.jsonlines"
eval_path = "test.english.jsonlines"
conll_eval_path = "test.english.v4_gold_conll"
lm_path = ""
genres = [
  "bc"
  "bn"
  "mz"
  "nw"
  "pt"
  "tc"
  "wb"
]
eval_frequency = 5000
report_frequency = 100
log_root = "logs"
log_dir = "logs/final"
Loading word embeddings from glove.840B.300d.txt...
Done loading word embeddings.
Loading word embeddings from glove_50_300_2.txt...
Done loading word embeddings.
2019-05-05 13:26:44.945108: W tensorflow/core/graph/graph_constructor.cc:1265] Importing a graph with a lower producer version 26 into an existing graph with producer version 27. Shape inference will have run different parts of the graph with different producer versions.
/home/khashab2/ideaProjects/e2e-coref/env3.6/lib/python3.6/site-packages/tensorflow/python/ops/gradients_impl.py:112: UserWarning: Converting sparse IndexedSlices to a dense Tensor of unknown shape. This may consume a large amount of memory.
  "Converting sparse IndexedSlices to a dense Tensor of unknown shape. "
2019-05-05 13:26:56.805005: I tensorflow/core/platform/cpu_feature_guard.cc:141] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
Restoring from logs/final/model.max.ckpt
Document text: Traceback (most recent call last):
  File "demo.py", line 50, in <module>
    print_predictions(make_predictions(text, model))
  File "demo.py", line 32, in make_predictions
    tensorized_example = model.tensorize_example(example, is_training=False)
  File "/home/khashab2/ideaProjects/e2e-coref/coref_model.py", line 138, in tensorize_example
    max_sentence_length = max(len(s) for s in sentences)
ValueError: max() arg is an empty sequence
The specs of the environment:
Python 3.6.7
Tensorflow: 1.12.0
Tensorflow-gpu: 1.12.0
Tensorflow-hub: 0.2.0
kentonl / e2e-coref

ValueError: max() arg is an empty sequence #56
