Sagemaker 'NoneType object' issue with data in 'walkthrough-classification-mxnet-sagemaker' example

I've been following the walkthough found [here](https://github.com/developmentseed/label-maker/blob/master/examples/walkthrough-classification-mxnet-sagemaker.md) (albeit with a smaller bounding box), and have initiated a Sagemaker Notebook instance. The data.npz file is sitting in the sagemaker folder, and I'm having no problem reading it when running the relevant sections of mx_lenet_sagemaker.py in a new notebook on the instance, however when I run the second cell of [SageMaker_mx-lenet](https://github.com/developmentseed/label-maker/blob/master/examples/nets/SageMaker_mx-lenet.ipynb) I hit the following error:

```
ValueError: Error training sagemaker-mxnet-2018-07-08-18-12-13-217: Failed Reason: AlgorithmError: uncaught exception during training: 'NoneType' object has no attribute 'read'
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/container_support/training.py", line 36, in start
fw.train()
File "/usr/local/lib/python2.7/dist-packages/mxnet_container/train.py", line 191, in train
model = user_module.train(**kwargs_to_pass)
File "/opt/ml/code/mx_lenet_sagemaker.py", line 92, in train
train_iter, val_iter = prep_data(data_path)
File "/opt/ml/code/mx_lenet_sagemaker.py", line 14, in prep_data
data = np.load(find_file(data_path, 'data.npz'))
File "/usr/local/lib/python2.7/dist-packages/numpy/lib/npyio.py", line 402, in load
magic = fid.read(N)
AttributeError: 'NoneType' object has no attribute 'read'
```
After several hours trying different fixes I'm having little to no luck debugging, but was hoping you could check the example to ensure it runs fine when you attempt it?