OSError: cannot identify image file <_io.BytesIO object at 0x7fc87f2ec678>

I'm following the [Example Use: A building detector with TensorFlow API](https://github.com/developmentseed/label-maker/blob/master/examples/walkthrough-tensorflow-object-detection.md), and ran into an error when trying to generate the training/eval data.

## System information
** What is the top-level directory of the model you are using: **
~/tensorflow/models/research/object_detection

**OS Platform and Distribution:** Linux Ubuntu 16.04, Elementary OS

**TensorFlow installed from (source or binary):** pip install tensorflow-gpu

**TensorFlow version:** v1.5.0

**Python version:** v3.6.0

**CUDA/cuDNN version:** CUDA 9.0, cudnn 7.0, nVidia driver: 384.111

**GPU:** nVidia GeForce GTX 1070 Ti 8GB memory

**CPU:** Intel x86-64 Intel Core i5-6400K @ 2.70GHz x 4, 16GB memory

**Exact command to reproduce:**

```
python tf_record_generation.py --label_input=labels.npz --train_rd_path=data/train_buildings.record --test_rd_path=data/test_buildings.record
```

**Sample code / logs**:

```
python tf_record_generation.py --label_input=labels.npz --train_rd_path=data/train_buildings.record --test_rd_path=data/test_buildings.record
You have 221 training tiles and 144 test tiles ready
Traceback (most recent call last):
File "tf_record_generation.py", line 172, in
tf.app.run()
File "/home/xban/.pyenv/versions/general/lib/python3.6/site-packages/tensorflow/python/platform/app.py", line 124, in run
_sys.exit(main(argv))
File "tf_record_generation.py", line 143, in main
tf_example = create_tf_example(group, train_dir)
File "tf_record_generation.py", line 56, in create_tf_example
image = Image.open(encoded_jpg_io)
File "/home/xban/.pyenv/versions/general/lib/python3.6/site-packages/PIL/Image.py", line 2572, in open
% (filename if filename else fp))
OSError: cannot identify image file <_io.BytesIO object at 0x7fc87f2ec678>
```