Indesign CS7 is Indesign CC, you need to save file from indesign cc as IDML then open in indesign cs6.
Good thing I didn't trash it
I'm not sure if the error message is updated to "Adobe InDesign (CC 9)" or something, if InDesign CS6 is updated to the latest version v8.0.2 …
You can examine a indd file from a script you can run with InDesign:
[Ann] Identify Your InDesign File