Several issues seem to be causing this. It seems to be any type of runtime error.
For example, if I include a non-existent module in the app, I'll end up getting a
*** Terminating app due to uncaught exception 'NameError' when the app is run, which is expected.
If I make the same error in a spec, all you get is
*nil description*. My guess is for some reason, runtime errors are not properly displayed.