title: Tensorflow Object Detection, error while generating tfrecord [TypeError: None has type NoneType, but expected one of: int, long]
body: <p>When checking across different solutions available on the net, most people (including datitran) pointed out that it might be a missing class or a misspell of a class in the train csv file. Am not able to figure that out since the labelling is done using labelImg, it saves these classes as xml, the xml_to_csv.py converts this to a csv. Am not sure under what circumstance I could have had the opportunity to miss out or misspel any class incorrectly.</p>
  
  <p>Here's the error am dealing with:</p>
  
  <pre><code>(OT) 
  nisxxxxx@xxxxxxxx:~/Desktop/OD/models/research/object_detection$ 
  python generate_tfrecord.py --csv_input=data/train_labels.csv  --
  output_path=data/train.record
  Traceback (most recent call last):
    File "generate_tfrecord.py", line 192, in &lt;module&gt;
      tf.app.run()
    File "/home/nisxxxxx/Desktop/test_OD/OT/lib/python2.7/site-
  packages/tensorflow/python/platform/app.py", line 48, in run
      _sys.exit(main(_sys.argv[:1] + flags_passthrough))
    File "generate_tfrecord.py", line 184, in main
  tf_example = create_tf_example(group, path)
    File "generate_tfrecord.py", line 173, in create_tf_example
      'image/object/class/label': 
  dataset_util.int64_list_feature(classes),
  File"/home/nishanth/Desktop/test_OD/models/research/object_detection/utils/dat
  aset_util.py", line 26, in int64_list_feature
      return 
  tf.train.Feature(int64_list=tf.train.Int64List(value=value))
  TypeError: None has type NoneType, but expected one of: int, long
  </code></pre>
  
  <p>Has anyone been able to solve this problem? </p>