PoS Tagging in QA Model

Question

my code:

import numpy as np

def pos_tag_fun(x_list, tag):
  tagged_sentences = []
  for sentence in x_list:
    tagged = nltk.pos_tag(sentence)
    tagged_sentences.append(tagged)
  
  print(tagged_sentences[0])


  # Extract the sentences and the corresponding POS tags
  train_x_list, train_tag_tags =[], []
  for tagged_sentence in tagged_sentences:
**    sentence, tags = zip(*tagged_sentence)
**    train_x_list.append(np.array(sentence))
    train_tag_tags.append(np.array(tags))

  print(tagged_sentences[0])
  print("Tagged sentences: ", len(tagged_sentences))
  print("Tagged words:", len(tag))
  
  print(train_x_list[0])
  print(train_tag_tags[0])

ques_list = pos_tag_fun(question_list, ques_tag)

getting error:

[('glacier', 'NN'), ('cave', 'VBP'), ('form', 'NN')]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-46-7073281060f4> in <cell line: 26>()
     24   print(train_tag_tags[0])
     25 
---> 26 ques_list = pos_tag_fun(question_list, ques_tag)

<ipython-input-46-7073281060f4> in pos_tag_fun(x_list, tag)
     13   train_x_list, train_tag_tags =[], []
     14   for tagged_sentence in tagged_sentences:
---> 15       sentence, tags = zip(*tagged_sentence)
     16       train_x_list.append(np.array(sentence))
     17       train_tag_tags.append(np.array(tags))

ValueError: not enough values to unpack (expected 2, got 0)

it is giving expected output if I run w/o the defining it as a function else it gives not enough value error. But when I print the output for tagged_sentence; it is a tuple of two items like this:

('glacier', 'NN')
('direct', 'JJ')
('apollo', 'NNS')
('long', 'JJ')
('beretta', 'NN')
('vul', 'NN')

I am confused why when calling the function it is giving this error.