Hi Nathan,
are your images square images or rectangle? I probably encountered the same problem a few days ago and found the solution in this thread. As the resizing method cuts off some parts of the images during transforms it can happen that the only parts with bboxes are cut off and then the error happens. When using SQUISH as resize method, it works (at least that di it for me).
And if you are using rectangle images, you have to pass the size of the images to get_data as well: for example get_data("bs",128) resizes to square images and get_data("bs", (80,128)) would be the size of some rectangle image. The size is passed as height x width.
Maybe this can help you fix your code, too 
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