title: Python Pandas NLTK Tokenize Column in Pandas Dataframe: expected string or bytes-like object
body: <p>I have the following sample data frame with a 'problem_definition' column:   </p>
  
  <pre><code>ID  problem_definition  
  1   cat, dog fish
  2   turtle; cat; fish fish
  3   hello book fish 
  4   dog hello fish cat
  </code></pre>
  
  <p>I want to word tokenize the 'problem_definition' column. </p>
  
  <p>Below is my code: </p>
  
  <pre><code>from nltk.tokenize import sent_tokenize, word_tokenize 
  import pandas as pd 
  
  df = pd.read_csv('log_page_nlp_subset.csv')
  
  df['problem_definition_tokenized'] = df['problem_definition'].apply(word_tokenize)
  </code></pre>
  
  <p>The code above gives me the following error: </p>
  
  <p>TypeError: expected string or bytes-like object</p>