title: Python KeyError when using pandas
body: <p>I'm following a tutorial on NLP but have encountered a key error error when trying to group my raw data into good and bad reviews. Here is the tutorial link: <a href="https://towardsdatascience.com/detecting-bad-customer-reviews-with-nlp-d8b36134dc7e" rel="nofollow noreferrer">https://towardsdatascience.com/detecting-bad-customer-reviews-with-nlp-d8b36134dc7e</a></p>
  
  <pre><code>#reviews.csv
  I am so angry about the service
  Nothing was wrong, all good
  The bedroom was dirty
  The food was great
  
  #nlp.py
  import pandas as pd
  
  #read data
  reviews_df = pd.read_csv("reviews.csv")
  # append the positive and negative text reviews
  reviews_df["review"] = reviews_df["Negative_Review"] + 
  reviews_df["Positive_Review"]
  
  reviews_df.columns
  </code></pre>
  
  <p>I'm seeing the following error:</p>
  
  <pre><code>File "pandas\_libs\hashtable_class_helper.pxi", line 1500, in pandas._libs.hashtable.PyObjectHashTable.get_item
  KeyError: 'Negative_Review'
  </code></pre>
  
  <p>Why is this happening?</p>