InDesign CC 2015 won't open 2017 .idml file

BobLevine, you're right -- it turned out to be a corrupt file. The client sent me a new one. Thanks for your reply.

To answer your question why do I use 2015 when my client uses 2017? If you really want to know, it's because I have many clients and most of them provide me with template files to work from. Some of them are still using CS6 because apparently it's difficult for large organizations to keep abreast of new software. I have to be able to work with whatever they give me. So why do I not upgrade myself to 2017 and use it when I receive files in 2017? Because I would first need to update my MacOS software (and you can criticize me for not doing that in a timely fashion, either) before I can install CC 2017. Why have I not updated my OS software? Because every time I do, it breaks the Distiller process (and countless other random apps and settings) that I'm required to use by the clients who are still using CS6. Why do they still insist on making PDFs from postscript files instead of exporting right from InDesign? I wish I knew. Again, I have to follow their process if I want to keep getting work from them.

TL; DR: It's a major hassle and workflow disruption to upgrade apps and system software and I avoid doing so for as long as possible.