title: TypeError: ufunc 'add' did not contain a loop with signature matching types (dtype('<U21'), dtype('<U21')) -> dtype('<U21')
body: <p>I run my code and it throw a error in line 79:</p>
  
  <blockquote>
    <p><code>numpy.core._exceptions.UFuncTypeError: ufunc 'add' did not contain a loop with signature matching types (dtype('&lt;U21'), dtype('&lt;U21')) -&gt; dtype('&lt;U21').</code></p>
  </blockquote>
  
  <p>Everybody know how to fix please help me. Thank you so much. 
  My code:</p>
  
  <pre><code>68 multi_df = pd.read_csv(FLAGS.VoTT_csv)
  69 labels = multi_df["label"].unique()
  70 labeldict = dict(zip(labels, range(len(labels))))
  71 multi_df.drop_duplicates(subset=None, keep="first", inplace=True)
  72 train_path = FLAGS.VoTT_Folder
  73 convert_vott_csv_to_yolo(
  74    multi_df, labeldict, path=train_path, target_name=FLAGS.YOLO_filename
  75 )
  
  76 file = open(classes_filename, "w")
  
  77 SortedLabelDict = sorted(labeldict.items(), key=lambda x: x[1])
  78 for elem in SortedLabelDict:
  79   file.write(elem[0] + "\n")
  80 file.close()
  </code></pre>